234 THE EVOLUTION OF SEX. 



cell-division, or has this as its necessary precedent, it is associated 

 with a katabolic crisis. 



§ 4. Argument from the Gradations between Asexual Sever- 

 ance of Parts and the Liberation of Special Sex- Cells. — Discuss- 

 ing asexual reproduction, we have noticed that some worm-types 

 break into two or more parts, which start new individuals. That 

 some nemerteans normally break up into pieces, as they do in 

 the feverish anxiety of capture, is most probable ; and this is 

 certainly the case in certain annelids. From a syllid, which 

 sets free a sexual individual, the overgrowth of an asexual parent, 

 to one which liberates a series of joints, or even a single joint, 

 bearing reproductive elements, is but a slight step. From the 

 last case, to the rupture which liberates sex-elements, is again 

 only a slight advance. A similar series is well illustrated among 

 the Hydromedusce. The breakage or thinning away which sets 

 a large portion free is a katabolic process, in a sense a local 

 death. The gentleness of the gradient warrants us in concluding 

 that the liberation of sex-cells, in its earlier expressions at least, 

 is associated with a local or with a general katabolic crisis. 



§ 5. Argumefit f'om the Close Connectio?i betiveen Reproduc- 

 tion and Death. — Without going back to primitive disintegra- 

 tions, or the asexual severance of more or less large portions, 

 we may point further to the close connection between reproduc- 

 tion and death, even when the former is accomplished by 

 specialised sex-cells. We shall presently discuss at greater 

 length this nemesis of reproduction, but it is important here to 

 emphasise that the organism not unfrequently dies in continu- 

 ing the life of the species. In some species of the primitive 

 annelid Polygordius, the mature females die in liberating 

 the ova. At a very different level, the gemmules of the 

 common fresh-water sponge are formed in the decay of the 

 asexual adult, while even the sexual summer forms, especially 

 the males, are peculiarly unstable and mortal. The whole 

 history of this form seems a continuous rhythm between life 

 and growth on the one hand, and death and reproduction on the 

 other. Or again, the flowering of phanerogams is often at once 

 the climax of the life and the glory of death. In his ingenious 

 essay on the origin of death, Goette has well shown how closely 

 and necessarily bound together are the two facts of reproduction 

 and death, which may be both described as katabolic crises. 



§ 6. Argument from E7ivironmental Conditions wJiich favour 

 Reproduction. — The rhythm between nutrition and reproduc- 



