214 THE EVOLUTION OF SEX. 



processes, and an organism in an environment where katabolism 

 is, at an early stage, likely to gain the ascendant. The former 

 is thus naturally asexual, the latter sexual. A survey, in fact, 

 of the conditions and characteristics of the two sets of forms, 

 inevitably leads us to regard the asexual generation as the ex- 

 pression of predominant anabolism, and the sexual as equally 

 emphatically katabolic. Alternation of generations is, in fine, 

 a rhythm between a relatively anabolic and katabolic prepon- 

 derance. 



§ 12. Ongin of Alternation of Generations. — Even in an individual 

 plant or animal there are vegetative and reproductive periods ; alternation 

 of generations involves the separation of these to different individuals, by the 

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

 asexual vegetative tendency preponderates ; in most medusoids, the sexual 

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

 in the pedigree of the organism. Thus Hreckel distinguishes a progressive 

 from a retrogressive origin ; in the former, the organisms are in transition 

 from preponderant asexual to sexual reproduction ; in the latter, the organisms 

 are returning or degenerating from dominant sexuality to an asexual pro- 

 cess. It is safe to say that the latter is more frequently the right inter- 

 pretation of the facts. So far as reproduction is concerned, one of those 

 medusoids {TrachymednsLZ) which have no corresponding hydroid parent, 

 or a jelly-fish like Pclagia which has no fixed asexual hydra-tuba stage, is 

 nearer the ancestral habit than those members of both divisions which 

 exhibit alternation of generations. ^Vhere we have alternating series of 

 similar forms with different degrees of sexuality, e.g.., the rhythm between 

 parthenogenesis and true sexual reproduction in aphides, Weismann once 

 interpreted the facts as associated with the periodic action of external influ- 

 ences (" 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, e.g., in the 

 midge Cecidojnyia ; {b) the case of the Hydromedusse, where sexuality is 

 postponed in early life, and asexual reproduction dominates ; and (c) an 

 origin from division of labour 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 con- 

 tinuous alternation between growth and multiplication, nutrition and repro- 

 duction, 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 

 Terminolog)' of the Reproductive Organs of Plants " (Proc. Liverpool Biol. 

 Soc, Vols. III. andTV.), we take the follo\Wng scheme : — 



A. Asexual stage or sporophyte, produces spores in sporangia {ovospor- 

 ajigia and sperniosporangia in higher Cryptogams and Phanerogams). 



B. Sexual stage ox gamophyte {oophyte and sper?nophyte where the thallus 

 is unisexual), produces ova and sperms in ovaries and sperviaries ; the pro- 

 duct of union of ovum and sperm being an oosperm. 



