122 Animal Life and Intelligence, 



CHAPTEE V. 



HEREDITY AND THE ORIGIN OF VARIATIONS. 



The law of heredity, I have said ahove, may be regarded 

 as that of persistence exempHified in a series of organic 

 generations. Variation results — it is clear that it must 

 result — from some kind of differentiating influence. Such 

 statements as these, however, though they are true enough, 

 do not help us much in understanding either heredity or 

 variation. 



Let us first notice that normal cases of reproduction 

 exemplify both phenomena — heredity with variation; 

 hereditary similarity to the parents in all essential respects, 

 individual variations in minor points. This is seen in 

 man. Brothers and sisters may present family resem- 

 blances among each other and to their parents, but each 

 has individual traits of feature and of character. Only in 

 particular cases of so-called "identical twins" are the 

 variations so slight as not to be readily perceptible by even 

 a casual observer. 



Now, when we seek an explanation of these well-known 

 facts, we may be tempted to find it in the supposition that 

 the character of the parents does not remain constant, that 

 the character influences the offspring, and that therefore 

 the children born at successive periods will differ from each 

 other, while twins born in the same hour will naturally 

 resemble each other. As Darwin himself says,* " The 

 greater dissimilarity of the successive children of the same 

 family in comparison with twins, which often resemble 

 each other in external appearance, mental disposition, and 



* " Animals and Plants under Domestication," vol. ii. p. 239. 



