IN THE THEOKY OF NATURAL SELECTION. 



upon the duration of the capacity : for force and matter undert 

 simultaneous increase, and are inseparably connected in this as ii. 

 all other instances. This theory does not, however, exclude the 

 possible occurrence of circumstances under which such an associa- 

 tion is no longer necessary. 



I could only consent to adopt the hypothesis of rejuvenescence, if 

 it were rendered absolutely certain that reproduction by division 

 could never under any circumstances persist indefinitely. But this 

 cannot be proved with any greater certainty than the converse pro- 

 position, and hence, as far as direct proof is concerned, the facts are 

 equally uncertain on both sides. The hypothesis of rejuvenescence 

 is, however, opposed by the fact of parthenogenesis ; for if fertilization 

 possesses in any way the meaning of rejuvenescence, and depends 

 upon the union of two different forms of force and of matter, which 

 thus produce the power of reproduction, it follows that we cannot 

 understand how it happens that the same power of reproduction 

 may be sometimes produced from one form of matter, alone and 

 unaided. Logically speaking, parthenogenesis should be as im- 

 possible as that either nitric acid or glycerine should separately 

 produce the effect of nitro-glycerine. The supposition has indeed 

 been made that in the case of parthenogenesis, one fertilization is 

 sufficient for a whole series of generations, but this supposition is 

 not only incapable of proof, but it is contradicted by the fact that 

 certain eggs which may develope parthenogenetically are also capable 

 of fertilization. If, in this case, the power of reproduction were suf- 

 ficient for development, how is it that the egg is also capable of 

 fertilization ; and if the power were insufficient, how is it that the 

 egg can develope parthenogenetically? And yet one and the same 

 egg (in the bee) can develope into a new individual, with or with- 

 out fertilization. We cannot escape this dilemma by making 

 the further assumption, which is also incapable of proof, that a 

 smaller amount of reproductive force is required for the development 

 of a male individual than for the development of a female. It is 

 true that the unfertilized eggs of the bee produce male individuals, 

 while the fertilized ones develope into females, but in certain other 

 species the converse association holds good, while in others, again, 

 fertilization bears no relation to the sex of the offspring. 



Although the mere fact that parthenogenesis occurs at all is, in 

 my opinion, sufficient to disprove the theory of rejuvenescence, it is 



