264 



THE EVOLUTION OF SEX. 



thus seen to be the obverse of the other. And if we group under the term 

 individuation all those race-preservative processes by which individual life is 

 completed and maintained, and extend the term genesis to include all those 

 processes aiding the formation and perfecting of new individuals, the result of 

 the whole argument may be tersely expressed in the formula — Individuation 

 and Genesis vary inversely. And from this conception im- 

 portant corolleries open ; thus, other things equal, advancing 

 evolution must be accompanied by declining fertility ; again, 

 if the difficulties of self-preservation permanently diminish, 

 there will be a permanent increase in the rate of multiplica- 

 tion, and conversely. 



In attempting the inductive verification of these a priori 

 inferences, practical difficulties arise, owing to the high com- 

 plexity of each of our two sets of factors and the independent 

 variability of their details, and thus the total cost of indi- 

 viduation and of genesis alike is hard of estimation and com- 

 parison. For this purpose, however, there are successively 

 to be investigated, — (1) the antagonism between growth and 

 genesis, sexual and asexual ; (2) that between development 

 and genesis ; (3) that between expenditure and genesis ; and 

 (4) the coincidence between high nutrition and genesis. It is 

 impossible to summarize the wealth of evidence drawn from 

 a wide survey of the animal and vegetable world contained 

 in the chapters devoted to those various heads, but attention 

 may be called to the last and most obscure of these. It is 

 indeed evident a priori that, if the cost of individuation be 

 once provided for, a higher nutrition will render possible a 

 greater propagation, sexual or asexual, and this may be 

 abundantly verified by observation and experiment. Witness 

 the case of aphides, in which the rate of parthenogenetic 

 reproduction is found to be directly proportional to tempera- 

 ture and food-supply ; or, again, that of domestic animals, 

 such as the sheep, whose fertility is in direct relation to rich- 

 ness of pasture and warmth of climate ; or, finally, and most 

 obviously of all, that of field or fruit crops, upon which the 

 influence of increased liberality of manuring will not be dis- 

 puted. Yet it is sometimes maintained, for both plants and 

 animals, that overfeeding checks increase, while limited nutri- 

 ment stimulates it ; and to support this view there are cited 

 such cases as that of the barrenness of a very luxuriant plant, 

 and the fruitfulness which appears on its depletion. But if 

 this objection really held, manuring would in all cases be 

 inexpedient, instead of only in plants where the growth of 

 sexless axes is still too luxuriant ; and a tree which has borne 

 a heavy crop should, by this depletion, bear again yet more 

 heavily, instead of being more or less barren next year unless manured. Or the 

 difficulty may also be met by interpreting such vegetative luxuriance, not as a 

 case of higher individuation at all, but simply as a case of asexual multipli- 

 cation of secondary axes ; or again, and perhaps most simply, by regarding the 

 appearance of sexual reproduction on depletion simply as a case of the previ- 

 ously demonstrated antagonism between genesis and growth. 



Fig. 100. — A species 

 of Onion with asex- 

 ual vegetative bul- 

 bils (b) among the 

 flowers (a). 



