CHAP, xix HEREDITY 363 



continuity 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 we call heredity 

 a ' Fate," it is plain that we 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 having been evolved there would 

 still be heredity. 



v 



1. 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 multicellular animal develops from an 

 egg-cell with which a male-cell has united in an intimate 

 way. The egg-cell supplies most of the initial 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. Each parent 

 usually contributes the same amount of nuclear material 



/ 



the same number of chromosomes to the offspring, 

 and this nuclear stuff is very essential. 



In some cases (few as yet known) half of the spermato- 

 zoa have the same number of chromosomes as the ovum, 

 while the others have one less. There is clear evidence 

 in some of these cases that an egg fertilised by a sperma- 

 tozoon with the same number of chromosomes develops 

 into a female, while an egg fertilised by the other type of 

 spermatozoon (with one chromosome less) develops 

 into a male. The extra chromosome, which half of the 

 spermatozoa have and half have not, is called the x-ele- 

 ment or accessory chromosome, and it seems as if the 

 presence of two x-elements in the fertilised ovum liberated 

 the quality of femaleness, while in the presence of a single 

 x-element maleness finds expression. 



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 or some 

 near relative, and the assertion does not arouse any sur- 

 prise, although the statement, often more obviously true, 



