MEANS AND METHODS OF EVOLUTIONARY CHANGE 331 



We have seen that the formation of geographic races (subspecies) repre- 

 sents one of the first steps in development of that diversity which is a hall- 

 mark of evolutionary change, and that formation of the greater diversity 

 characterizing species seems to be a later step in the same process. 



All of these evolutionary manifestations require explanation. It is one 

 thing to note their occurrence, another to explain the means by which they 

 occur. Although the manifestations are varied, explanation of them ob- 

 viously has a common denominator. In every case we are called upon to 

 explain diversity — how it arises and how it is preserved and perpetuated. 

 For evolution is the process by reason of which descendants differ from 

 their ancestors. 



We now direct our attention to the various factors contributing to the 

 production of change in animals and hence to the origin of diversity. In 

 Chapter 2 we sketched broadly some of the principal factors involved. In 

 this chapter and the following ones we shall discuss these in more detail 

 and introduce others not mentioned previously. Rereading of Chapter 2 

 will form appropriate introduction to the subject matter of these chapters. 



SHUFFLING THE GENES 



It is common observation that no two individuals are 

 alike. Even so-called "identical twins," if observed closely enough, will be 

 found to differ in some respects. And the extreme diversity among the pop- 

 ulation at large needs no emphasizing. The same diversity exists in the 

 subhuman portions of the animal kingdom. If "every mouse looks like every 

 other mouse'' to us it is because we have not observed mice closely enough 

 to be familiar with their distinguishing features. Recall the comment made 

 by many Americans that all Chinese look alike, and the similar comment 

 of Chinese newly come to this country that Americans are difficult to recog- 

 nize because they all look alike. The individual differences are there; the 

 difficulty in recognizing them lies in faulty observation, based on inadequate 

 past experience. 



What is the basis of this diversity among individuals? Insofar as it is of 

 hereditary nature it arises in good part as the result of the operation of the 

 mechanisms of inheritance. 



On an earlier page (p. 10) we mentioned the units of heredity called 

 genes. These are contained in chromosomes found in all cells composing 

 the body. The chromosomes occur in pairs, one member of each pair being 

 inherited from the father, the other from the mother. When germ cells are 

 formed by an individual the members of each pair of chromosomes separate 



