THE EVOLUTION OF SEX 15 



The appearance of sex cells (gametes) is but one step in the 

 evolution of sex. It represents the attainment of sexuality, 

 but the process becomes much more highly developed. 



14. Isogamy. — When gametes first appear, in some such 

 way as has been described, the two which fuse seem to be 

 exactly alike. They resemble each other in size and activ- 

 ity, and in every structure which can be distinguished. 

 This fact is indicated by the word isogamy, which means 

 " similar gametes," and those plants whose pairing gametes 

 are similar, as Ulothrix, are said to be isogamous. 



The act of fusing of similar gametes is usually called 

 con jugation , which means a " yoking together " of similar 

 bodies. Of course it is a sexual process, but the name is 

 convenient as indicating not merely the process, but also an 

 important character of the gametes. The sexual spore 

 which results from this act of conjugation is called the 

 zygote or zygospore, meaning " yoked spore." 



In isogamy it is evident that while sexuality has been 

 attained there is no distinction between sexes, as obtains in 

 the higher plants. It may be called a unisexual condition, 

 as opposed to a bisexual one. The next problem in the 

 evolution of sex, therefore, is to discover how a bisexual 

 condition has been derived from a unisexual or isogamous 

 one. 



15. Heterogamy. — Beginning with isogamous forms, a 

 series of plants can be selected illustrating how the pairing 

 gametes gradually became unlike. One of them becomes 

 less active and larger, until finally it is entirely passive and 

 very many times larger than its mate (Fig. 7). The other 

 retains its small size and increases in activity. The pairing 

 gametes thus become very much differentiated, the larger 

 passive one being the female gamete, the smaller active one 

 the male gamete. This condition is indicated by the word 

 heterogamy, which means " dissimilar gametes," and those 

 plants whose pairing gametes are dissimilar are said to be 

 heterogamous. 



