HEREDITY AND EVOLUTION 89 



very long before the body of the young human being 

 can be readily discriminated from that of the young 

 puppy. 4 



Taking next the mature organism which is the 

 parent life,/ there is within this fully-developed body, 

 a great y/triety of cells, only a limited number of 

 which ar/3 reproductive cells. These last constitute a 

 distinct- order, each having its own individual life, de 

 pendent, however, for its continued vitality on its place 

 in the parent life, and liable to modification according 

 to changes in that life. These reproductive cells, con 

 taining the promise of the corning generation, depend 

 for fertilisation on the union of male and female 

 elements. These in vital combination start the new 

 life-history. That the nucleus of the male cell 

 (spermatozoon) penetrates to the nucleus of the 

 female cell (ovum), has been ascertained. But 

 observation has not supplied data warranting further 

 inference, as to distinctive hereditary potency coming 

 from each parent. With an organism so minute, 

 and movements so difficult to watch, it is doubtful 

 whether the data are likely to be found. But it 

 seems clear from the facts of after life, that heredi 

 tary influence from both parents can be traced in the 

 life of the offspring. 



In thus briefly stating the characteristics common to 

 the germ-cell of all species, including its structure and 

 functions, the ernbryological testimony for continuity 

 in organic evolution is seen to be striking, As the 

 seeds of the several plants are distinct in Nature, so 

 are the germ-cells of the several animals. From each 

 fertilised ovum there is developed, by progression 



1 Man s Place in Nature, p. 67. 



