i g8 



THE EVOLUTION OF SEX. 



and the sexual as equally emphatically katabolic. Alternation of 

 generations is, in fine, a rhythm between a relatively anabolic and 

 katabolic preponderance. 



XII. Origin of Alternation of Generations. — Even in an individual plant or 

 animal there are vegetative and reproductive periods ; alternation of genera- 

 tions involves the separation of these to different individuals, by the interpola- 

 tion of more or less asexual reproduction. In most hydroids, the asexual 

 vegetative tendency preponderates ; in most medusoids, the sexual repro- 

 ductive dominates. But the origin in each particular case is involved in the 

 pedigree of the organism. Thus Haeckel distinguishes a progressive from a 

 retrogressive origin ; in the former, the organisms are in transition from pre- 

 ponderant asexual to sexual reproduction ; in the latter, the organisms are 

 returning or degenerating from dominant sexuality to an asexual process. It 

 is safe to say that the latter is more frequently the right interpretation of the 

 facts. So far as reproduction is concerned, one of those medusoids ( Trachy- 

 medusce) which have no corresponding hydroid parent, or a jellyfish like 

 Pelagia which has no fixed asexual hydra-tuba stage, is nearer the ancestral 

 habit than those members of both divisions which exhibit alternation of 

 generations. Where we have alternating series of similar forms with different 

 degrees of sexuality, for example, the rhythm between parthenogenesis and 

 true sexual reproduction in aphides, Weismann once interpretated the facts as 

 associated with the periodic action of external influences ("Studies in the 

 Theory of Descent," chap. v.). But in contrast to such cases he distinguished, 

 {a) an origin from metamorphosis, where one stage in the life-history becomes 

 precociously reproductive, for example, in the midge Cccidomyia ; (b) the case 

 of the Hydromeduscr, where sexuality is postponed in early life, and asexual 

 reproduction dominates ; and (c) an origin from division of labor within a 

 colony. Without entering upon a discussion of each case in relation to its 

 history and environment, it is not possible to do more than reassert that in 

 many different degrees the continuous alternation between growth and multi- 

 plication, nutrition and reproduction, asexuality and sexuality, anabolism and 

 katabolism, may express itself in the life-history of the organism. 



Postscript.— From Mr. R. J. Harvey Gibson's valuable paper on "The 

 Termynology of the Reproductive Organs of Plants" (Proc. Liverpool Biol. 

 Soc, Vols. III. and IV.), we take the following scheme : — 



A. Asexual stage or sporophytc produces spores in sporangia {ovosporangia 

 and spermosporatigia in higher Cryptogams and Phanerogams). 



B. Sexual stage or gamophyte zoophyte and spcrmophyte where the thallus 

 is unisexual), produces ova and sperms in ovaries and spermaries ; the product 

 of union of ovum and sperm being oosperm. 



