326 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



The fact that different methods of reproduction may obtain in 

 different colonies of the same species, although with thoroughly 

 identical habits, may depend upon differences in the external con- 

 ditions (as in Bosmina and C/iydorus mentioned above), or upon the 

 fact that the transition from sexual to parthenogenetic reproduction 

 is not effected with the same ease and rapidity in all the colonies 

 of the same species. As long as males continue to make their 

 appearance in a colony of Apus, sexual reproduction cannot wholly 

 disappear. Although we are unable to appreciate, with any degree 

 of certainty, the causes by which sex is determined, we may never- 

 theless confidently maintain that such determining influences may 

 be different in two widely separated colonies. As soon, however, as 

 parthenogenesis becomes advantageous to the species, securing its 

 existence more efficiently than sexual reproduction, it will not only 

 be the case that the colonies which produce the fewest males will gain 

 advantage, but within the limits of the colony itself, those females 

 will gain an advantage which produce eggs that can develope without 

 fertilization. When the males are only present in small numbers, it 

 must be very uncertain whether any given female will be fertilized : 

 if therefore the eggs of such a female required fertilization in order 

 to develope, it is clear that there would be great danger of entire failure 

 in this necessary condition. In other words: as soon as any females 

 begin to produce eggs which are capable of development without 

 fertilization, from that very time a tendency towards the loss of 

 sexual reproduction springs into existence. It seems, however, that 

 the power of producing eggs which can develope without fertiliza- 

 tion is very widely distributed among the Arthropoda. 



APPENDIX VI. W. K. BROOKS' THEORY OP HEREDITY 1 . 



The only theory of heredity which, at any rate in one point, 

 agrees with my own, was brought forward two years ago by W. K. 

 Brooks of Baltimore 2 . The point of agreement lies in the fact that 

 Brooks also looks upon sexual reproduction as the means employed 

 by nature in order to produce variation. The manner in which he 

 supposes that the variability arises is, how r ever, very different from 



1 Appendix to page 277. 



3 Compare W. K. Brooks, ' The Law of Heredity, a Study of the Cause of Variation, 

 and the Origin of living Organisms.' Baltimore, 1883. 



