THE EVOLUTION OF SEX 23 



they become closely united and exchange 

 nuclear elements, after which they separate 

 again. In other cases, the ordinary forms 

 produce special elements (spores) which con- 

 jugate, and these are sometimes dimorphic 

 macrospores and microspores corresponding 

 U> the eggs and sperms of higher animals. 



In the life history of the common bell- 

 animalcule, Vorticella (see Fig. 3), a small- 

 sized free-swimming individual attaches itself 

 to and bores into another individual, of 

 normal size, which remains moored by its 

 contractile stalk to the water-weed. What 

 have we here but male and female in minia- 

 ture? It is true, of course, that most of the 

 Protozoa remain strictly unicellular (or per- 

 haps one should say non-cellular), so that 

 they can only be analogues of males and females 

 among multicellular creatures. They corres- 

 pond, indeed, not so much to higher organisms, 

 as to the sex-cells of higher organisms ; they 

 are like ova and spermatozoa which have 

 not formed " bodies." But the important 

 fact is that even among the Protozoa we find 

 examples of special reproductive units which 

 combine in pairs, and are dimorphic macro- 

 spores and microspores. For here we are at 

 the beginning of sex. 



THE CASE OF VOLVOX. The second step 

 on the ladder is best illustrated by Volvox, 

 an Infusorian colony, not uncommon in clear 

 ponds, which shows us a body in the making. 

 It is a beautiful green ball of a thousand or 

 ten thousand flagellate cells, according to the 

 species, bound to one another by bridges of 

 protoplasm. It rolls itself quickly through 



