THE GENERAL SCHEME 



always the same, and there are puzzling and obscure cases 

 awaiting solution. At any rate the situation is clear in the 

 higher animals, which always reproduce by the union of an 

 egg cell with a sperm cell. Just why the whole animal king- 

 dom, except a few of the lowest and simplest creatures, settled 

 down so completely to the egg-and-sperm system, we can only 

 guess. Perhaps in some early ancestral animal, not originally 

 bisexual, it happened that some of the reproductive cells 

 were unusually well stored with nutritive substances. This 

 would be an advantage, for it would help to tide the embryos 

 over the earliest stages of their development before they 

 could feed themselves. Such cells would, however, be slug- 

 gish, and the chance of two of that kind meeting would 

 therefore be reduced. A germ cell that happened to be lighter 

 and more mobile would be more likely to meet the relatively 

 sluggish cell. Once started, such a trend toward two types 

 would progress and become fixed, by the familiar Darwinian 

 process of survival of the fittest, and ultimately we should 

 arrive at the characteristic arrangement, namely large eggs 

 laden with food (yolk) and small active sperm cells. 



Fio. 7. Sperm cells of various animals and man. Greatly magnified. 



{ 15 } 



