2o6 



THE EVOLUTION OF SEX. 



If we do emphasise the "protonema" stage (/), and regard the moss- 

 plants as asexually budded from it, the formula runs : — 



rpO- 



In the fern, the vegetative sexless generation was the more conspicuous ; 

 in mosses, the sexual generation. In a way this recalls the contrast 

 between the life-history of many a zoophyte, and that of the common 

 jelly-fish aurelia. The asexual hydroid colony is more conspicuous than 

 the usually small s^vimming-bell, but the sexual jelly-fish is much more 

 conspicuous than the minute asexual "hydra-tuba." The common com- 

 parison between medusoid and hydroid on the one hand, and prothallus 

 and fern-plant on the other, is rather misleading, simply because the 

 hydroid merely buds off the medusoid, while the fern-plant produces the 

 prothallus from a special reproductive cell or spore. In some ferns and 

 mosses, however, a more exact parallel is occasionally exhibited. The 

 production of " spores " may be suppressed, and from the place where they 

 should be formed a (sexual) fern-prothallus or a new (sexual) moss-plant is 

 vegetatively developed, just as medusoid from hydroid. This exceptional 

 occurrence is technically called apospory. The very opposite of this also 

 occurs, the suppression not of the spore -bearing, but of the sexual genera- 

 tions. The fern-plant then arises vegetatively from the prothallus ; and 

 this would be paralleled if we supposed the sporocyst of the fluke to bud 

 off rediae (as it sometimes does), and these to continue the species without 

 ever becoming really sexual, solely by means of the special cells above 

 described. 



§ 4. Co77ibination of both these Alternations. — The asexual hydroid buds 

 off a medusoid, the fertilised ovum of which develops into a hydroid. 

 Here there is simple alternation between sexual and asexual reproduction. 



A sexless fern-plant forms special reproductive cells (spores), which 

 develop parthenogenetically into a sexual prothallus, from the fertilised 



egg-cell of which the fern-plant arises. 



