92 EVOLUTION AND MAN S PLACE IN NATURE 



in the growth of the organism in which they arise, 

 and their chief association with the other cells of the 

 body is that certain of the latter are of service in their 

 nutrition. l These germ-cells are thus distinct centres 

 of organic life, whose specific function is reproduction 

 of the species. The laws of heredity are the laws 

 of the activity of these germ-cells, as that may be 

 affected by the vitality and functional activity of the 

 mature body in which they have their sphere of exist 

 ence. When biological advance is such as to involve 

 difference of sex, there is the blending of two elements 

 in the new life, amphimixis, from the two parents. 

 The act of impregnation may be described as the 

 fusion of the ovum and spermatozoon ; the most im 

 portant feature in this act appears to be the fusion 

 of a male and female nucleus. 2 Forthwith the 

 combined elements work in unison, involving pro 

 vision for double agency in the line of heredity. 

 The after progress may be that of the larva, en 

 compassed in food-yolk, or that of the embryo 

 nourished in the womb. At the other extreme of 

 observation, we are most familiar with the fact that 

 in the history of families there is in family like 

 ness the reappearance of the characteristics of both 

 parents. 



Here, then, are the known conditions presenting a 

 problem of great perplexity. A single cell, out of the 

 millions of diversely differentiated cells which compose 

 the body, becomes specialised as a sexual cell ; it is 

 thrown off from the organism, and is capable of 

 producing all the peculiarities of the parent body, in 



1 Sir W. Turner s Address : Nature, p. 527. 



2 Comparative Embryology, by F. M. Balfotir, i. p. G9. 



