824 ECOLOGY 



genesis is advantageous in that it eliminates the disadvantages of alter- 

 nating generations. The elimination of sexual fusion, though often 

 regarded as a sign of degeneracy, is quite as likely to be a sign of pro- 

 gressive evolution. Furthermore, the theory which holds that sexuality 

 leads to variability has little support from the facts of parthenogenesis, 

 since no plant genera are more variable than are Taraxacum and 

 Hieracium. 



Concluding remarks. So far, at any rate, as the seedless plants 

 are concerned, the significance of sexual reproduction is in doubt, as 

 has been indicated in the preceding paragraphs. The obvious advan- 

 tages appear to be subsidiary, and not at all commensurate with the 

 amount of energy and material that is involved. The appearance of 

 dioecism, together with that of alternating generations and of heteros- 

 pory, multiplies disadvantages and introduces no conspicuous corre- 

 sponding advantages, unless it should be discovered that amphimixis 

 is inherently advantageous ; in this event dioecism, alternating genera- 

 tions, and heterospory are highly beneficial, since they increase the 

 chance of fusion between gametes that differ in immediate ancestry. 

 In the seed plants, through the marked subordination of the game- 

 tophytic generation, through the retention of the megaspore, and 

 through the dispersal of the embryo (seed), the chief disadvantages of 

 alternation and heterospory are eliminated. To a small extent the dis- 

 advantages of alternation are eliminated through apogamy, apospory, 

 and parthenogenesis, but the elimination of disadvantage has come 

 chiefly through vegetative reproduction, which in the great majority of 

 plants insures the perpetuation of species, regardless of the presence or 

 absence of sexual reproduction. The almost unlimited capacity for 

 vegetative reproduction in the gametophyte generation of bryophytes 

 and of the sporophyte generation of ferns and seed plants doubtless 

 has been the means of preserving many species that otherwise would 

 have perished. Thus it is not to be assumed that the progress of 

 evolution necessarily is advantageous, and that heterospory and alterna- 

 tion must be an improvement over homospory and lack of alternation. 

 Probably the decadence of the heterosporous pteridophytes and of many 

 groups of animals is due to disadvantageous trends in evolution. Even 

 in the seed plants, supremacy is due, not so much, probably, to heteros- 

 pory and alternation, as to various features which eliminate their disad- 

 vantages and most of all to their high capacity for vegetative reproduc- 

 tion, for foliage display, and for the development of secondary wood. 



