CHAP. XX Heredity 321 



between successive generations, choosing this definition 

 because it is misleading to talk about "heredity" as a 

 " basal principle in evolution," as a " great law," as a 

 "power," or as a "cause." When I call heredity a 

 " Fate," it is plain that I speak fancifully, but " principle " 

 and " law" are dangerous words to play with. We cannot 

 think of life without this organic relation between parents 

 and offspring, and had species been created instead of being 

 evolved there would still be heredity. 



I. The Facts of Heredity. — An animal sometimes 

 arises as a bud from its parent, and in rare cases from an 

 egg which requires no fertilisation, but apart from these 

 exceptions, every animal develops from an egg-cell with 

 which a male-cell has united in an intimate way. The 

 egg-cell supplies most of the living matter, but the nucleus 

 of the fertilised egg-cell is formed in half from the nucleus 

 of the immature ovum, in half from the nucleus of the 

 spermatozoon. Let us emphasise this first fact that each 

 parent contributes the same amount of nuclear material to 

 the offspring, and that this nuclear stuff is very essential. 



Another fact is more obvious, the offspring is very like 

 its kind. One of the first things that people say about an 

 infant is that it is like its father or its mother, and the 

 assertion does not arouse any surprise, although the truer 

 verdict that the infant is like any other of the same race is 

 received with contempt. But every one admits that " like 

 begets like." 



This likeness between offspring and parent is often far 

 more than a general resemblance, for peculiar features and 

 minute idiosyncrasies are frequently reproduced. Yet one 

 must not assume that because a child twirls his thumbs 

 in the same way as his father did the habit has been 

 inherited. For pecuUar habits and structures may readily 

 reappear by imitation, or because the offspring grow up in 

 conditions similar to those in which the parents lived. 



Abnormal as well as normal characters, " natural " to 

 the parents, may reappear in their descendants, and the list 

 of weaknesses and malformations which may be transmitted 

 is long and grim. But care is required to distinguish 



V 



