270 Evolution and Adaptation 



what is quantitively the rate at which it is working and has 

 worked." This statement expresses no more than Pearson's 

 conviction that the process of evolution has taken place by 

 means of selection. He ignores other possibilities, which if 

 established may put the whole question in a very different 

 light. 



Heredity and Continuous Variation 



It has been to a certain extent assumed in the preceding 

 pages that both parents are alike, or, if different, that they 

 have an equal influence on the offspring. This may be true 

 in many cases for certain characteristics. Thus a son from 

 a tall father and a short mother may be intermediate in 

 height, or if the father is white and the mother black, the 

 children are mulattoes. But other characters rarely or never 

 blend. In such cases the offspring is more like one or the 

 other parent, in which case the inheritance is said to be 

 exclusive. Thus if one parent has blue eyes and the other 

 black, some of the children may have black eyes and others 

 blue. There are also cases of particular inheritance where 

 there may be patches of color, some like the color of one 

 parent, some like that of the other parent. The latter two 

 kinds of inheritance will be more especially considered in the 

 subsequent part of this chapter ; for the present we are here 

 chiefly concerned with blended characters. 



How much in such cases. does each parent contribute to the 

 offspring ? This has been expressed by Galton in his law of 

 ancestral heredity. This law takes into account not only the 

 two parents, but also the four grandparents, and the eight 

 great-grandparents, etc. There will be 1024 in the tenth 

 generation. These 1024 individuals may be taken as a fair 

 sample of the general population, provided there has not been 

 much interbreeding. Are we then to look upon the individ- 

 ual as the fused or blended product of the population a few 

 generations back ? If this were true, should we not expect 



