290 THE SIGNIFICANCE OF SEXUAL REPKODUCTION 



which sexual reproduction has been altogether lost, and in which 

 parthenogenesis is the only form of propagation. In the animal 

 kingdom, such a condition chiefly occurs in species of which the 

 closely-allied forms exhibit the above-mentioned alternation between 

 parthenogenesis and amphigony, viz. in many Cynipidae and Aplil<la<\ 

 and also in certain freshwater and marine Crustacea. We may 

 imagine that these parthenogenetic species have arisen from forms 

 with alternating methods of reproduction, by the disappearance of 

 the sexual phase. 



In any particular case, it may be difficult to point out the motive 

 by which this change has been determined ; but it is most probable 

 that the same conditions which originally caused the intercalation 

 of a parthenogenetic stage have been efficient in causing the 

 gradual disappearance of the sexual stage. If a species of Crust- 

 acean, with the above-described alternating method of reproduction 

 (heterogeny), were killed off by its enemies on a larger scale than 

 before, it is obvious that the threatened extinction of the species 

 could be checked by the attainment of a correspondingly greater 

 degree of fertility. Such increased fertility might well be produced 

 by pure parthenogenesis (see Appendix V, p. 323), by means of 

 which the number of egg-producing individuals in all the previous 

 sexual generations would be doubled. 



In a certain sense, this would be the last and most extreme 

 method by means of which a species might secure continued 

 existence, for it is a method for which it would have to pay 

 very dearly at a later period. If my theory as to the causes of 

 hereditary individual variability be correct, it follows that all species 

 with purely parthenogenetic reproduction are sure to die out ; not, 

 indeed, because of any failure in meeting the existing conditions 

 of life, but because they are incapable of transforming themselves 

 into new species, or, in fact, of adapting themselves to any new 

 conditions. Such species can no longer be subject to the process 

 of natural selection, because, with the disappearance of sexual re- 

 production, they have also lost the power of combining and in- 

 creasing those hereditary individual characters which they possess. 



All the facts with which we are acquainted confirm this con- 

 clusion, for whole groups of purely parthenogenetic species or 

 genera are never met with, as would certainly be the case if 

 parthenogenesis had been the only method of reproduction through 



