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THE EVOLUTION OF SEX. 



of the apparently asexual offspring. In this he partially prophesied 

 the modern conception of a residual persistent germ-plasma. Soon 

 afterwards Leuckart attempted to treat all as cases of metamorphosis, 

 thereby greatly extending the meaning of that term. The labors of 

 some of the foremost naturalists have both extended Steenstrup's 

 observations and rendered them more precise. We now know that the 

 phenomenon is of wider occurrence than was at first supposed, and also 

 that the title has been unduly extended to cover several entirely differ- 

 ent sets of facts. It is necessary, therefore, to notice the various forms 

 which the rhythm of reproduction may take. 



II. The Rhythm between Sexual and Asexual Reproduc- 

 tion.— The clearest case to start with is that of many hydroids. A 

 sessile plant-like zoophyte, which buds off numerous nutritive persons, 

 produces in the warm months modified individuals which are set 

 adrift as medusoid persons. Unlike the hydroid which bore them, 

 these become sexual; and from their fertilized ova and embryo 

 develops, which eventually settles down to start a new sessile colony. 

 And thus through the seasons we have hydroid asexually producing 

 sexual medusoids, and these again producing hydroids. The life- 

 history for two complete rhythms may be written in the formula, 

 in which M, F, and A stand for male, female and asexual forms 

 respectively, — 



M - A - M — A - M 

 S A F A F 



Fig. 62.— A, asexual hydroid; S, sexual medusoid; fertilized ova at base. 



Or take, in slight contrast, the life-story of the common jellyfish 

 Aurelia. Large free-swimming sexual animals produce ova which are 

 fertilized by sperms; the embryo develops, not however into a jelly- 

 fish, but into a sessile hydroid-like organism or "hydra-tuba." By 

 growth and division this asexually produces the jellyfish in turn. 

 Here the sexual generation is more stable and conspicuous, the 

 reverse of the former case, but the same formula applies. 



Or take a case from another class of animals, the marine worms. 

 Some of the sylids have the following life-history. A worm remains 

 asexual, never attaining either the external or internal organs of the 

 sexual individuals. It gives rise to these, however, by an asexual 



