Department of Biology and Horticulture

General Biology II                                    Name ______________________

Dr. Louis Crescitelli                                 Date  __________ Sect. ______

Scientific Investigation in Biology

I.      Introduction

Purpose

The purpose of this exercise is to help Biology students to understand the scientific basis of Biology.  It will improve their ability to successfully complete investigations in the Biology laboratory by providing the necessary background on scientific principles and methods.  Understanding the scientific principles that guide research in Biology will enable students to better understand the science of Biology and derive more benefit from the direct experience and practical knowledge provided by the laboratory experience.

Biology is a science course, and as such operates on the basis of scientific principles and methods.  An important prerequisite for understanding the scientific basis of Biology is to know the definitions of Science and Biology.    

Science is the process of inquiry that asks questions about the natural world and endeavors to provide answers and explanations for them by using an empirical approach based upon direct observation, experience, and testing, typically by experimentation.  The tested and verified evidence produced by this process increases our understanding and knowledge of the composition, organization, energy, forces, operation, and life comprising the physical reality of the universe. 

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  • Biology is the scientific study of living things.  Biology is one of many sciences.  It is classified as a natural science.  Other natural sciences include Chemistry, Physics, Geology, Astronomy, Earth Science, Meteorology, Oceanography, as well as a number of others.

    II.     Objectives

    1.    To consider the fundamental importance of Science and the Scientific Method in generating knowledge in the field of Biology.

    2.    To learn the definition of Science and understand how Science differs from other fields of study.

    3.    To learn the definition of Biology and examine the relationship of Biology to the other natural sciences.

    4.    To prepare students to succeed in the Biology laboratory by providing them with the scientific background on science and the scientific method required for laboratory work.

    5.    To learn the steps in the Scientific Method using examples from everyday life and a scientific investigation.

    6.    To learn the characteristics of a good hypothesis and become aware that a hypothesis cannot be proven to be true.

    7.    To know what is meant by a controlled experiment and understand its importance in Biology.

    8.    To understand the significance of the experimental and control groups that are used in a controlled experiment.

    9.    To examine the types of variables in an experiment:  the experimental variable, the dependent variable, and controlled variables.

    10.  To identify the types of data that are collected during an experiment: observations and results, and be able to distinguish between observations and inferences.

    11.  To learn the importance of a conclusion.

    12.  To realize the importance of verifying findings in scientific research.

    13.  To understand that the term theory is used differently in Science than it is in everyday language.  In Science, it is a well-tested concept not a conjecture.

    14.  To understand the limitations of the Scientific Method.

    III.    Science and the Scientific Method

    Scientists ask questions as they make observations and think about the composition and operation of the natural world, and they try to answer them by designing tests.  They form predictions, known as hypotheses about the outcome of the tests and then perform experiments to see if their predictions are correct.  The information gathered in this way is subjected to a process of verification, and if the findings remain valid after repeated testing, and are found to be sufficiently important, they may develop into theories and laws.  Science differs from other fields of study.  The scientific approach distinguishes Science from other fields of knowledge such as Philosophy, History, English, Literature, Religion, etc.  The knowledge developed in Science must be testable and withstand a process of verification.  Science can only deal with the natural world.  Things that cannot be measured or analyzed such as supernatural spirits or ghosts cannot be studied by Science.

    The Process of Science

    Science can be done in various ways.

    We are about to begin a discussion of the Scientific Method, which will cover the basic principles of how Science is done.  The Scientific Method involves testing hypotheses by carrying out experiments.  It is the traditional way of explaining how Science is done and is in fact an important way of gaining knowledge and organizing scientific papers.  But before we begin our discussion on the Scientific Method, it is important to point out that the Scientific Method is not the only way Science is done. Science can actually be done in a number of ways.  For example, in the field of Animal Behavior, studies are typically conducted primarily by observing animals in the wild and recording their behavior rather than doing experiments.  Important discoveries may happen by chance, such as the discovery of penicillin by Alexander Fleming.  Fleming was doing a routine task of going through and discarding contaminated Petri dishes when he made a key observation that led to the discovery of penicillin.  In some cases, important scientific discoveries have been made based upon results that were not expected by the scientist.  An example of this is Frederick Griffith’s discovery of transformation in bacteria.  Griffith’s discovery was made while he conducting experiments that he originally intended would produce a vaccine that would protect people from pneumonia.  However, while he was performing a control experiment he had an unexpected result that led to the discovery of bacterial transformation instead.    Watson and Crick discovered the molecular structure of DNA.  However, Watson and Crick did not make the discovery as would be conventionally done by following the steps of the Scientific Method.  Their approach to solving the molecular structure of DNA consisted of building a model of DNA.  We will revisit the examples given above and explain them more fully in subsequent sections of the study guide.

    Steps in the Scientific Method

    The process used to produce knowledge in Science is described, as the Scientific Method.  The Scientific Method is a systematic and formalized set of procedures used by scientists to develop new knowledge and explain natural phenomenon.    It is based on a system of inquiry, observation, forming hypotheses, and testing hypotheses by way of experiments.  The Scientific Method is made up of a number of steps, which are outlined below.

    1.    Stating the Problem

    The problem is a question to be answered.

    2.    Gathering information about the problem.

    This is generally done by going to the library or going online and looking up all the scientific papers that are pertinent to the problem.

    3.    Forming the Hypothesis.

    A hypothesis is a prediction of the outcome of an experiment.  It is sometimes called an educated guess because it is based on library research. 

    4.    Test the Hypothesis.

    The hypothesis is tested by performing an experiment.

    5.    Collect Data

    Data includes observations and results.  Observations are descriptions.  Results are derived from a measurement.

    6.    Form Conclusions.

    A conclusion is a summary of the data and states whether it supports or contradicts the hypothesis.

    7.    Theory

    A theory is an explanation of an occurrence in the natural world that has been verified based upon substantial observation and experimentation.

    8.    Law

    A scientific law describes an occurrence in the natural world and is based upon observation and experimentation.  Unlike a theory, a law doesn’t explain the phenomenon.

    IV.    Use of the Scientific Method in Everyday Life

    Although we may not realize it, the Scientific Method is utilized to solve problems that arise in everyday life activities.  Some examples of everyday life activities that incorporate the use of the Scientific Method are given below.  Using one of the examples below, identify the steps of the scientific method that are used to solve the problem.

    Example 1 

    A person goes out to his car in the morning and it will not start when he turns the key.  The engine does not turn over and the lights will not operate.  The person reads a car repair manual to study possible causes of the failure of the car to start.  He guesses that the car will not start because of a dead battery.  He tests the battery and finds that it will not hold a charge.  He replaces the battery, and the car starts. 

    Example 2 

    A person purchases a new plant and places it in a location inside her home.  After several days, she notices that the leaves are yellow and that the plant does not seem to be growing well.  A friend who is a gardener suggests several reasons why the leaves are turning yellow.  The person who bought the plant supposes that the plant is not receiving enough light.  She relocates the plant next to a window with more light.  After two weeks the leaves are dark green.  

    Example 3 

    The staff of Consumer Reports Magazine wanted to choose the best make and model of the available refrigerators.  They read brochures and manuals and checked online for information about the refrigerators from the manufacturers.  They obtained refrigerators representing all of the most popular brands.  They tested them on the basis of cooling, capacity, energy efficiency, reliability, and cost.  They collected and analyzed their data.  After comparing the performance of all of the refrigerators, they decided that a Whirlpool refrigerator was the best and placed it at the top of their ratings.

    V.    Illustration of the Scientific Method in a Biological Investigation

    Steps in the scientific method

    1.  Stating the Problem

    We humans seem to have an innate curiosity about ourselves and our surroundings.  As we observe and explore the natural world around us we often come up with questions about how and why things work.  In order to be tested as part of a scientific investigation, the question must be about something that is real, something that is observable, measurable, and testable.  Scientists try to explain natural phenomena by performing an inquiry, which is the process of finding answers to questions.  An inquiry is an approach to learning that provides explanations of questions, leads to greater understanding, and increases our knowledge of the natural world.   An inquiry is equivalent to a scientific investigation or research.

    Our question about the natural world could become the objective of a scientific study.  By asking a question, the investigator established the reason for a scientific research project.  It is to answer that question.  If the question is to be examined scientifically, it must be framed in a certain way.  In order to be tested scientifically, the question must be specific and about something that is real, observable, measurable, and testable.  In Science, a question posed to begin the investigation is known as the problem.

    The problem is a question to be answered such as “What is the effect of vitamin B-12 on the rate of growth in rats?”

    Remember that the problem should be stated as a question.  A problem in science is different than a problem in everyday life.  If you are asked to state the problem given the example of a car not being able to start, the problem is not “The car will not start”, but “Why won’t the car start?”  The statement of the problem in a scientific way as a question is the first step in the solution of a problem, not just a statement of fact.  If you are asked to give an example of a problem in Science you should say not, “Exercise causes an increase in heart rate,” but “What is the effect of exercise on heart rate?”

    Problems may arise from a number of sources.  The problem may arise as the result of an observation.  A problem may also be developed as a result of reading research papers.  The research may have suggested new questions to be explored or left some unanswered questions that could form the basis for a new study.  

     

    2.  Gather Information about the Problem

    Perform library research.  Look up scientific reports that pertain (apply) to your problem.

    The scientist attempts to find out everything that he can about the problem by reviewing the scientific literature pertaining to his problem.  This is done in libraries and/or online.

    3.  Forming the Hypothesis

    Hypothesis: A tentative statement about the natural world leading to deductions that can be tested (Steering Committee on Science and Creationism, National Academy of Sciences, 1999).

    A hypothesis is a prediction that can be tested experimentally to answer questions arising from scientific problems.  Developing a good hypothesis as outlined below enables an investigator to devise effective experiments to test the hypothesis, to determine whether it is valid, and in the process, gain an answer to his question about the scientific problem.  The hypothesis establishes the rationale for an experiment.  A hypothesis attempts to predict the outcome of an experiment.  Specifically, a hypothesis attempts to predict the effect of an experimental treatment, known as an experimental variable on what is being measured, the dependent variable in an experiment.

    Characteristics of a Good Hypothesis

    Education Based

    A hypothesis is often defined as an educated guess.  The hypothesis is an educated guess because it is constructed on the basis of previous observations, library research, online research, and other information obtained when the scientist gathered information about the problem.  Armed with this information, the scientist is prepared to devise possible solutions to the problem.

    Testable

    A hypothesis must be testable.  It must deal with real, observable, phenomena in the natural world that can be measured and analyzed. The investigation that follows from the hypothesis must be concerned with measurable evidence.

     
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    The Hypothesis is Falsifiable

    The hypothesis must be potentially falsifiable, that is, the hypothesis must be tested in a way that allows for the possibility that it may be proven false.  We cannot prove that a hypothesis is true in an absolute sense, but we can accept the hypothesis tentatively if it is supported by the data.  Obviously, the greater the number of times the experiment is repeated and consistently shows similar results supporting the hypothesis the more confidence we can have in it.  In practice the hypothesis accepted if it has withstood critical and repeated testing, especially if it conforms to accepted scientific theory.  Even when confirmed by a number of experiments, the hypothesis has been established as true only for the particular circumstances prevailing in the experiments.  We cannot guarantee that we have tested the hypothesis under every possible circumstance.  However, we accept the hypothesis.  But if in the future an experiment produces conclusive results showing that the hypothesis is false, we must reject it. 

    The Hypothesis must be Specific and Well-Defined

    The hypothesis must be as specific and well-defined as possible.  The reason for this is that the hypothesis is intended as the basis for designing an experiment to answer the question posed by the problem.  If the hypothesis is too broad, we would not be able to design an experiment to answer the question.  For example, the hypothesis “A poor diet is harmful to health” is not a good hypothesis.  Before an experiment can be designed and carried out, the hypothesis must be made more specific.  What is meant by a poor diet?  Is it a deficiency of a particular nutrient, or mineral, or vitamin?  Is it too much of a certain food?  What is meant by the phrase “harmful to health”?  Does it mean that the diet can lead to starvation or obesity?  Does it mean that it can weaken the skeleton or teeth?  Does the diet contain substances that can cause cancer?  A better hypothesis might be:  A deficiency in iron will decrease red blood cell production in the rat.  This specifies a particular dietary problem, identifies a particular aspect of health or body function, and names a particular animal.

    The Hypothesis May Become a Theory

    If the hypothesis withstands extensive testing, and explains an important observation in nature, it may be elevated to the status of a theory.  Even though a hypothesis has been extensively tested, it is still considered tentative.  The fact that hypotheses and theories in Science are tentatively accepted and are subjected to revision or rejection is not a weakness of Science, it is in fact a strength because it means that the findings have been critically examined and tested repeatedly and extensively under a variety of circumstances and never falsified.  However, they are always accepted tentatively and subject to further scrutiny and testing.  If an experiment is performed that conclusively falsifies the hypothesis, the hypothesis must be rejected.  This process, known as verification, is an essential part of the scientific process.

    The Null Hypothesis

    A null hypothesis is the prediction that the experimental treatment will have no effect.

    4.  Testing the Hypothesis

    An Experiment is a test of a hypothesis.

    Let’s assume that our hypothesis is that injections of vitamin B-12 will increase the growth rate of rats.  We will test this hypothesis by performing an experiment.

    In the experiment, the experimenter determines the effect of an experimental variable on what is being measured, the dependent variable in the experimental group.

    Variables in the Experiment

    An experiment involves three different kinds of variables:

    Experimental (Independent) variable – The factor that the experimenter manipulates to determine its effect as it is tested upon on one or more observed and measured variables in the experimental group.  It is equivalent to the experimental treatment.

    Dependent variable – The factor that is observed and measured to detect possible changes resulting from the effect of the experimental variable.  It is called the dependent variable because it “depends” upon the values of the experimental variable.

    Controlled variables – factors that must be kept constant.

    In our experiment the experimental variable is the vitamin B-12 treatment.  It will be given to the experimental group.    The dependent variable is the growth rate of the rats.  It is the variable that is measured.  Controlled variables include factors that could affect the experiment other than vitamin B-12 treatment.  These include temperature, food given to the rats, water, lighting, size of the cage, sex of the rats, etc.  There can be only one experimental variable in an experiment.  If there were more than one, we would not be able to determine which one caused the effect.  Although there can only be one experimental variable in the experiment, there may be several dependent variables.  For example, to measure growth, one could measure the weight of the rats as well as determine their head to tail length.  The experimental variable is also known as the independent variable.  An independent variable is a variable with values that are independent of changes in the values of other variables. 

    Groups in the Experiment

    As we have seen above, there are many factors that could affect what we are measuring in the experiment, the growth rate of the rats or the dependent variable.  We are interested in determining the effect of one variable, the vitamin B-12 treatment, the experimental variable.  How can we isolate the experimental variable while neutralizing the effect of the other variables?  It can be done by setting up a controlled experiment.

    In a controlled experiment, there are two groups: the experimental group, and the control group.

    1)    The experimental group receives the factor that is being tested.

    2)    The control group is a group of organisms that is made as similar to the experimental group as possible and is treated in exactly the same way except that it does not receive the factor that is being tested.

    In our experiment, the experimental group is a group of rats that is treated with vitamin B-12.  The control group is a group of rats that is maintained under exactly the same conditions as the experimental group except that it does not receive the treatment of vitamin B-12.  All of the other factors, the temperature, the food, the light, the cages etc. must be kept exactly the same on both the experimental and control groups.  These variables are known as control variables.  In a properly set up control experiment, the only variable that is different in a comparison between the two groups is the experimental variable (vitamin B-12).  Because the controlled variables are acting equally on both groups, their effect is canceled out.  The control group serves as a standard for comparison.  During the experiment the experimenter will measure the growth rate of the rats in both the experimental group and the control group.  The results for the two groups will be analyzed and compared.  If the experiment has been properly constructed, any difference in the growth rate of the rats in the experimental group versus the control group can be attributed solely to the one variable that is different, the effect of the vitamin B-12 treatment.

    Use of a Placebo

    In the experiment to determine the effect of vitamin B-12 on the growth rate of rats, if you inject animals in the experimental group with vitamin B-12, you must also inject the rats in the control group as well.  However, rather than receiving vitamin B-12, the rats in the control group are injected with a substance that does not affect the rate of growth,   such as injections of saline solution.  This is known as a placebo.  The reason for this is that apart from the substance that is injected (vitamin B-12), the injection by itself could affect the growth rate of the animals.  If the animals in the experimental group received injections and the animals in the control group did not, the findings of the experiment could be called into question.  Assuming that the experiment demonstrated that vitamin B-12 caused an increase in the growth of rats, another scientist reading the report and critical of the findings could say that the effect was caused by the injections and not the action of the vitamin B-12 treatment.  Because the experiment was set up incorrectly and the rats in the experimental group received injections but those in the control group did not, two experimental variables would be acting in the experiment and it would not be possible to determine which one, the vitamin B-12 or the injections caused the effect.

    Size of Groups

    The larger the number of organisms in the experimental and control groups, the better.  Obviously, experiments are limited by costs, availability of space, etc.  However, the experimenter should use the largest sample size that is feasible.  Gathering data from a larger sample size averages out any inconsistencies or variations that might be present.  Additional support is provided for the hypothesis.  The experimental and control groups are closer to the theoretical population in the world that they are intended to represent.

    Randomization, Elimination of Bias

    The groups are usually formed by “randomization,” that is to say, by assigning individuals to one group or the other by drawing lots or by some means that does not involve human discrimination.  Every attempt should be made to eliminate sources of bias during the collection of data.  Many studies are conducted using a “double-blind” procedure.  A “double blind” study is an experimental procedure in which neither the subjects of the experiment nor the experimenters know whether the treatment being administered is the experimental variable or a placebo, that is whether the subjects are part of an experimental group or a control group.  This approach limits subjectivity and bias on the part of the experimenter.  A triple blind study is one in which neither the subjects, nor the person administering treatment, nor the person evaluating the response to the treatment knows which patients are receiving the experimental treatment and which are receiving placebos

    Methods and Materials

    When the scientist publishes his findings in a scientific paper, he will take care to report exactly what materials were used.  Usually he will not only report the precise name of the chemical and the concentrations used, but will also report the name of the chemical company from which the chemical was purchased.  He will also try to be as complete and accurate in describing the methods used to perform the experiment as possible.  This is to ensure that another scientist who wishes to verify the experiment can repeat it in exactly the same way as it was originally performed.

    5.  Collect Data

    Data consists of observations and results.

    An important part of the investigation is to make careful observations.  For example, if one were dealing with a chemical reaction as part of the investigation, one would look for and report color changes in the test tube or the formation of a precipitate.

    An observation is a description rather than a measurement.

    A result is derived from a measurement.

    Examples of results are the weights of animals or the lengths of leaves.  The investigator must report the results accurately and honestly.

    Graphs and tables are often used to summarize the data.  They allow readers of the scientific paper to quickly grasp the meaning of the data and to see trends.

    Difference between Observations and Inferences

    An observation is a description made using the five senses.

    An inference is an explanation or interpretation of an observation.

    Developing an inference is a creative process.

    For example: 

    Observation:    The grass on the front lawn has turned yellow.

    Inference:        The grass is dry and has to be watered.

    Observation:  Karl von Frisch trained bees by feeding them on blue paper.  After training they will land on blue paper free of food, even though it is in a different location and covered with a glass plate so that their sense of smell is blocked.  He found that bees can also see white, yellow, blue, and violet.  If the colored card is set in the middle of a set of gray-toned cards, bees can distinguish the colored card.

    Inference:  Bees have color vision.

     
     
     
    Observation:  The stem and leaves of the plant are facing in the direction of the sun.

    Inference: (3 points) ________________________________________________________________________________________________________________________________________________________________________________________________________________________

    6.  Forming Conclusions

    Following the careful recording of observations and results, the data must be analyzed.

    Importance of Statistics

    Statistical analysis is generally an essential part of the analysis and interpretation of data. 

    Statistical tests such as the “T-test” is applied to determine whether a significant difference exists between the experimental and control groups.  Statistics cannot be used to prove that a particular generalization is true.

    Make Conclusions

    A conclusion is a generalization and inference based upon the data.

    A conclusion is an interpretation of the results and a determination of the outcome of the experiment.  It is a statement of whether the hypothesis was supported or falsified.

    We say “supports the hypothesis” because the hypothesis cannot be proven to be true, it can only be supported by the results of the experiment.  However, a hypothesis can be proven to be false.

    Importance of Replicating Experiments

    After the experiment has been completed, it should be repeated.  At a minimum, there should be three replications.  When the experiments are repeated, they should be set up and conducted exactly like the original experiment.  If the results of the repeated experiment are the same as in the original experiment, additional support is provided for the hypothesis.

    Verification: Publishing and Peer Review 

    When the experimenter completes the experiment, he usually publishes his findings in a scientific journal.  In this way, other scientists become aware of the findings.  They are also given an opportunity to verify the validity of the findings by repeating the experiment.  This process of peer review and verification is an important part of the scientific process.  After a finding has been subjected to a process of verification and has been accepted, we can have a greater degree of confidence in it.

    7.  Theory

    Eventually, if all the evidence continues to support the new idea, it may become widely accepted and become a theory.

    Notice that scientists do not use the word “theory” as the general public does.  To many people a theory is a highly tentative statement.  But when scientists use the word theory, they imply that it has a very high degree of probability and that they have great confidence in it.

    Theory: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses (Steering Committee on Science and Creationism, National Academy of Sciences, 1999).

    A theory is supported by the facts and helps order and explain those facts.  Many scientific theories, such as the cell theory and the theory of evolution are so well supported by all known facts that they themselves are “facts” in the nonscientific application of that term.

    No theory in science is ever absolutely and finally proven.

    8.  Laws

    Law: A descriptive generalization about how some aspect of the natural world behaves under stated circumstances (Steering Committee on Science and Creationism, National Academy of Sciences, 1999).

    Example: Mendel’s Laws

    VI.    Limitations of the Scientific Method

    It is important to realize that knowledge in Science is not generated exclusively by using the Scientific Method.  In addition to making scientific discoveries in the laboratory by using the Scientific Method, scientific knowledge can be generated in other ways.  For example, a scientific study could be descriptive, that is, based upon a series of observations.  The sequencing of the human genome is an example of this type of study.  So are many studies in the field of Animal Behavior, in which animals are observed in their natural environment.          

                Important discoveries may happen by chance, such as the discovery of penicillin by Alexander Fleming.  When Fleming made the discovery that led to the development of penicillin, he was not performing an experiment with this intent in mind.  Instead he was doing a routine laboratory task, which was discarding contaminated plates.  On one of the plates, Fleming noticed a clear zone between bacteria growing on the plate and a mold that had contaminated the plate.  He hypothesized that some substance had diffused outward from the mold that inhibited the growth of the bacteria.  This observation led to the discovery of penicillin.  Or, an experiment may produce an unexpected finding that may be very important.  An example of this is Frederick Griffith’s discovery of transformation in bacteria.  His was trying to produce an effective vaccine for pneumonia.  Instead, he found that living rough-strain bacteria could be changed into smooth-strain bacteria by a substance that had diffused into the living rough–strain cells from the nonliving smooth strain cells.  In this way, he discovered something that he did not expect, the process of transformation.

    Although the controlled experiment is the best device we have to test a hypothesis, it is not the only way to produce scientific knowledge.  For example, Watson and Crick discovered the molecular structure of DNA.  In doing so, they did not perform any controlled experiments in the laboratory.  Their approach to solving the molecular structure of DNA consisted of building a model of DNA.  In accomplishing this goal, they used data produced by Rosalind Franklin, Maurice Wilkins, Erwin Chargaff, and many other investigators. 

    Using the Scientific Method does not automatically guarantee the solution of a scientific problem.  It does not insure that an intelligent, creative, and insightful approach will be used in the scientific investigation to solve a problem.  It does not supply the intelligence necessary to ask a good question to be answered in an investigation.  It does not supply the ingenuity necessary to design meaningful experiments.  We have seen that scientific discoveries can be produced in a number of ways.  Even though scientific knowledge can be produced in other ways, and using it cannot guarantee an important discovery, the Scientific Method remains a powerful method of solving problems, planning scientific research projects, organizing the findings of scientific studies and reporting the results of scientific studies.  The other requirements must be supplied by the scientist.

    References:

    1.    Beveridge, W. I. B.  The Art of Scientific Investigation.  New York, NY:  Vintage Books A Division of Random House; 1957.

    2.    Frisch, Karl von.  The Dance Language & Orientation of Bees.  Cambridge, Massachusetts:  The Belknap Press of Harvard University Press; 1967.

    3.    Helms, Doris R., Carl W. Helms, Robert J. Kosinski, and John R. Cummings.  Biology in the Laboratory.  New York, NY:  W.H. Freeman and Company; 1998.

    4.    Montagu, Ashley.  Science and Creationism.  New York, NY:  Oxford University Press; 1984.

    5.    Centers for Disease Control and Prevention.  Growth Charts.  Available from:  http://www.cdc.gov/growthcharts/clinical_charts.htm

    6.    National Vital Statistics Reports, Vol. 63, No. 7, November 6, 2014.  Table 19. Estimated life expectancy at birth in years, by race, Hispanic origin and sex: Death-registration States, 1900-28, and United States, 1929-2010.  Available from:  http://www.cdc.gov/nchs/data/nvsr63/63_07.pdf

    7.    Steering Committee on Science and Creationism, National Academy of Sciences.  Science and Creationism:  A View from the National Academy of Sciences.  2nd edition.  Washington, DC:  The National Academies Press; 1999.

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