30 Heredity and Eugenics 



few and comparatively large protoplasts, they are swim- 

 ming spores (Fig. 3, a and b), and each can reproduce. If 

 the divisions continue, however, and result in numerous 

 and comparatively small protoplasts, they are unable to 

 reproduce (Fig. 3, c). However, if they come together in 

 pairs and fuse (Fig. ^, d), thus making one cell (protoplast) 

 out of two (Fig. 3, e), the new cell can reproduce. This 

 fusion is the sexual act, and the fusing cells are the sexual 

 cells (gametes). It is of interest to note that this first 

 appearance of sex is quite disconnected with the multipli- 

 cation of individuals. Individuals are multiplied through- 

 out the growing season by the spores. Toward the close 

 of the season, the gametes begin to appear, and the fusion 

 cells (zygotes) formed by their pairing develop heavy walls 

 that protect them through the unfavorable season (as the 

 winter). All the other structures of the plant perish, and 

 it exists through the winter only in the form of zygotes. 

 At the beginning of the next season, the zygotes produce 

 new plants, and these are multiplied by spores. The 

 service rendered by the sex act in this case, therefore, is to 

 produce a protected cell, which can carry the plant through 

 an unfavorable period; in short, the service is protection 

 rather than multiplication. 



The next advance in the evolution of sex was its differ- 

 entiation. The gametes at first are similar in appearance 

 and in behavior, but it must be recognized that this optical 

 test would be unable to detect any differences in quality. 

 It is upon the basis of appearance that such gametes are 

 said to be unisexual, which only means that they cannot 

 be distinguished as male and female. A series can be 

 arranged to illustrate a gradual differentiation of gametes 

 into two forms, unlike in appearance and in behavior. In 



