90 EVOLUTION 



reproductive organs. It is obviously at this 

 level, and not with the highly specialized sex 

 dimorphism of peacock and peahen, ruff and 

 reeve, lion and lioness, man and woman, that 

 the problem should be first studied. 



The problem is partly solved by considering 

 the simplest expressions of the sex-difference, 

 as we see it, for instance, in Volvox, an inter- 

 esting colonial Infusorian, which well illustrates 

 a body in the making. It is a beautiful rolling 

 ball of ciliated cells, and these component 

 units are connected by protoplasmic bridges. 

 From the ball of cells reproductive units are 

 sometimes set adrift, which divide to form 

 other colonies without more ado. But in 

 other conditions, when nutrition is checked, a 

 less direct mood of reproduction occurs. Some 

 of the cells in the ball become large, well-fed 

 elements the ova; others, less anabolic, fade 

 from green to yellow, divide and re-divide into 

 many minute units the spermatozoa. The 

 large cells of one colony are fertilized by 

 the small cells from another. Here we see 

 the formation of dimorphic reproductive cells 

 in different parts of the same organism. But 

 we may also find Volvox balls in which only 

 ova are produced, and others in which only 

 sperms are produced. The former seem to be 

 more vegetative and nutritive than the latter; 



