IN THE THEORY OF NATURAL SELECTION. 291 



a successional series of species. We always find it in isolated 

 instances, and under conditions which compel the conclusion that 

 it has become predominant in the species in question, and has not 

 been transmitted from any preceding- species. 



There still remains a very different class of facts which, so far as 

 we can judge, are in accordance with my theory as to the signi- 

 ficance 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 trans- 

 mitted 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 i not transmitted 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 this explanation, we shall have 

 to start from the supposition that new forms are not only created 

 by natural selection, but are also preserved by its means. In 

 order that any part of the body of an individual of any species may 

 be kept at the maximum degree of development, it is necessary 

 that all individuals possessing it in a less perfect form must be 

 prevented from propagation they must succumb in the struggle 

 for existence. I will illustrate this by a special instance. In 

 species which, like the birds of prey l , depend for food upon the 

 acuteness of their vision, all individuals with relatively weak eye- 

 sight must be exterminated, because they will fail in the competition 

 for food. Such birds will perish before they have reproduced them- 

 selves, and their imperfect vision is not further transmitted. In 

 this way the keen eyesight of birds of prey is kept up to its 

 maximum. 



But as soon as an organ becomes useless, the continued selection 

 of individuals in which it is best developed must cease, and 

 a process which I have termed panmixia takes place. When this 



1 I here make use of the same illustration which I employed in my first attempt 

 to explain the effects of panmixia. Compare the second Essay ' Qn Heredity.' 



u a 



