v.] IN THE THEORY OF NATURAL SELECTION. '>^7^^y 



modes of reproduction is in itself a sufficient proof of the 

 validity of the conclusions which have just been advanced. 



The fact that different methods of reproduction may obtain in 

 different colonies of the same species, although with thoroughly 

 identical habits, may depend upon difierences in the external 

 conditions (as in Bosmina and Chydonts 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 o{ 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 nevertheless confidently 

 maintain that such determining influences may be different in 

 two widely separated colonies. As soon, however, as partheno- 

 genesis 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 w^ithin 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 develop- 

 ment without fertilization, from that ver}'' 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 fertilization is very widely distributed among 

 the Arthropoda. 



Appendix VI. W. K. Brooks' Theory of Heredity '. 



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 ^ The point of agreement lies in 



^ Appendix to page 284. 



- Compare W. K. Brooks, 'The Law of Heredity, a Study of the 

 Cause of Variation and the Origin of Hving Organisms.' Baltimore, 1883. 



