298 SIGNIFICANCE OF SEXUAI REPRODUCTION [V. 



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 pa}' 

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

 hereditary individual variabilit}'^ 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 adapt- 

 ing themselves to any new conditions. Such species can no 

 longer be subject to the process of natural selection, because, 

 with the disappearance of sexual reproduction, they have also 

 lost the power of combining and increasing those hereditary 

 individual characters which they possess. 



All the facts with which we are acquainted confirm this 

 conclusion, 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 a successional series of species. We always find it in 

 isolated instances, and under conditions which compel the con- 

 clusion that it has become predominant in the species in ques- 

 tion, and has not been transmitted from any preceding species. 



There still remains a very dift'ercnt class of facts which, so 

 far as we can judge, are in accordance with ni}' theory as 

 to the significance of sexual reproduction, and which may be 

 quoted in its support. I refer to the condition of functionless 

 organs in species with parthenogenetic reproduction. 



Under the supposition that acquired characters cannot be 

 transmitted — and this forms the foundation of the views here 

 set forth — organs which are of no further use cannot become 

 rudimentary in the direct and simple manner in which it has 

 been hitherto imagined that degeneration takes place. It is 

 true that an organ which does not perform any function exhibits 

 a marked decrease of strength and perfection in the individual 

 which possesses it, but such acquired degradation is not trans- 

 mitted to its descendants, and we must therefore look for some 

 other explanation of the firmly established fact that organs do 

 become rudimentary through a series of generations. In seeking 



