CHAPTER XIII 

 INTRODUCTORY STATEMENT 



THE NATURE AND SCOPE OF GENETICS 



The validity of the general principle of evolution rests on the 

 kind of evidence presented in previous chapters. Volumes could be 

 written giving further evidence of the same sort. Very few thoughtful 

 persons, once confronted with these evidences, fail to be convinced as 

 to the reality of evolution. 



It is one thing to know that evolution has occurred and has fol- 

 lowed certain courses, but quite another thing to understand what 

 forces underly the process. A going process must have causes, and 

 it is our purpose in this section of the course to present an account of 

 what we know, or what we think we know, about the causes of organic 

 evolution. 



Our method of study is one that depends on the validity of the 

 doctrine of uniformitarianism. Exactly as in the science of geology 

 the method of investigation is that of studying in detail the changes 

 goiig on toda} r , of assuming that present changes are of the same na- 

 ture as changes in the past, and that the past may be interpreted in 

 terms of what we discover about the present. Thus, long series of 

 generations of rapidly breeding animals and plants are studied in- 

 tensively over periods of years, some species having been bred for over 

 twenty-five years, or for at least seven hundred generations, a period 

 equivalent in number of generations to 25,000 years of human life. 

 Hundreds of millions of individuals have been passed in review before 

 the keenly trained eyes of an army of competent investigators, all on 

 the lookout for the slightest change from the normal. Any observed 

 change is then followed through subsequent generations to determine 

 whether it is hereditary and how it affects the success of individuals 

 possessing it. Studies of this sort, together with examinations of the 

 germ cells to see whether or not correlated changes have occurred in 

 their materials, and, mathematical calculations of the relative fre- 

 quencies with which different characters occur in combination with 

 one another, have led to an understanding of the mechanism of evolu- 

 tion far more complete and detailed than anyone a decade or so ago 

 could possibly have hoped to attain. 



The experimental and analytical study of the processes and mech- 



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