250 GEORGE JOHN ROMANES 1888- 



extended so as to include failure to propagate from 

 any cause, besides that of death, through comparative 

 mal-adaptation in the struggle for existence. In this 

 extended sense, of course, it includes, as you say, 

 sexual selection, and also a variety of other prin- 

 ciples, which would thus fail to be distinguished 

 as distinct principles. Moreover, by thus identifying 

 them, the most distinguishing feature of natural 

 selection becomes obscured ; for, I take it, this most 

 distinguishing feature is not that of homogamy (or 

 breeding of like with like to the exclusion of breeding 

 with unlike), but homogamy where likeness is deter- 

 mined by adaptation. All the other forms of dis- 

 criminate breeding (such as sexual selection) agree 

 among themselves and with natural selection in 

 being exclusive, or in allowing propagation only 

 to some individuals of the species (i.e. those which 

 resemble one another in respect to the characters 

 which determine permission to propagate). But the 

 enormous difference between them all and natural 

 selection consists in the latter alone making for 

 improvement of type in respect of adaptation. Here 

 alone homogamy is due to a struggle for existence, is 

 brought about by death of the non-propagating, and 

 has reference to fitness in a life-preserving sense. 

 Surely this is the great distinguishing mark of 

 natural selection, considered as a principle in organic 

 evolution : not the merely exclusive breeding, which 

 is presented also by all the other principles which 

 your extension of the term would embrace, but which 

 in their case can have no effect in the way of 



