304 DARWINISM AND THE PROBLEMS OF LIFE 



size. The yellow of the hen's egg, for instance, is at 

 first a single cell while it is still inside the hen. The 

 nucleus of this cell is small, but the cell body takes up 

 an immense amount of yolk, and then the whole is 

 surrounded by the white and the shell ; these are later 

 products, superadded to the yellow. When the egg is 

 laid, it is, of course, fertilised, and there have already 

 been a number of cell cleavages, so that the white dot 

 on the yellow yolk, the " scar " as it is called, is really 

 a somewhat advanced embryo. 



Continuous adaptation has brought about great differ- 

 ences in the form of the spermatozoon and ovum in the 

 various animals. It is obvious that those animals will 

 always be favoured by selection whose germ-cells were 

 the surest to find each other ; animals whose sperma- 

 tozoa, for instance, were not active enough to reach the 

 female ova could not reproduce, and their sluggish 

 sperm-cells would die with them. Thus we can under- 

 stand why in the case of organisms that eject their seed 

 into water, it is produced in large quantities and is very 

 mobile. In these cases it is largely a matter of chance 

 whether a single spermatozoon will find its ovum. 

 When it does reach one, the physico-chemical nature 

 of the two cells attracts them to each other and fuses 

 them together. Even then it is no light task for the 

 mammal spermatozoa to pass up the oviducts to the 

 ovum in the ovary, and this explains their number and 

 activity. 



However, I have not space to deal with either the 

 great diversity of form in the animal ova and sperm- 

 cells or the interesting relation in which we always find 



