CH. LIX.] HEREDITY 859 



of opinion prevails. It does, however, appear to be pretty well 

 accepted that the material of the nucleus of the male and female 

 reproductive elements is of special importance in the transmission 

 of hereditary characters. That the chromosomes of the nucleus are 

 of vital importance is clearly shown by several facts. For instance, 

 they are constant in number not only in the cells of the body but 

 in all individuals of any particular species of animal or plant, 

 though differing in number in different species. The equal halving 

 of each chromosome, which occurs during mitosis, maintains this 

 numerical constancy, except in a certain period of the life history of 

 each individual, and this period occurs in the formation of the 

 reproductive cells (often called gametes). During one step in the 

 karyokinetic cell-division, half of the normal number of chromo- 

 somes are thrown out, and the act of fertilisation consists in the 

 fusion of the male and female gametes; each parental nucleus 

 provides half the normal number of chromosomes, and thus the 

 fertilised egg-cell starts with the full complement once more. A 

 large number of biologists regard the chromosomes as the actual 

 bearers of the characters which an organism inherits from its 

 parents, or, at any rate, adopt this view as a working hypothesis. 

 This places the transmission of characters from parent to offspring 

 upon a material foundation. Weismann's view that characters 

 acquired after birth are not transmissible rests mainly upon negative 

 evidence, and so cannot be considered as fully proved. 



The development of the two sexes took place fairly early in the 

 history of the animal world, but there are many animals of the 

 simplest kind in which no differentiation of sex occurs. The 

 determination of sex, that is, why a mother should bear at one time 

 a male, at another time a female infant, is a question of great 

 importance, and numerous theories have been advanced to explain 

 it. In certain invertebrate animals there is some evidence that sex 

 is preformed either in the ovum or the spermatozoon, or in both ; 

 that is to say, there are male and female ova, and male and female 

 spermatozoa which exhibit slight differences of structure; the 

 question of sex of the offspring will then depend upon which 

 element predominates after union. Others hold that potentialities 

 of producing either sex are present in all ova and all spermatozoa, 

 and that the ultimate sex is determined by some relationship which 

 is at present unknown, but which in all probability operates at the 

 time of fertilisation. 



The original Darwinian doctrine, just stated in outline, has since 

 been modified in several directions as research has progressed. The 

 work of Weismann just alluded to is important. But of all the 

 theories which have been grafted upon the original theory, that of 

 Mendel is perhaps of the greatest interest ; it certainly appeals to 



