'^^^S of 



brought 



"!' 



). 



Ti ■ 



p. 



^^erefor, 



"° sort of ; 



ers 



niemb 



" '°"^P«^^ed^it J 



I 



\ 



^^ ^^^ form and 



'cninHuencedbjsc 

 Pnt!y of the fad t 



revail in determ-' 



.1 ^1 



^ 



^ ^1 



hemcromorpliSjte 



1 vegetal form 



manner which is 



( 



I 



rate as a 



ffi. 



* 



zi 



incr his docW 



expre 



ssesit-' 



ccies are 



born » 



in tv>^ '% 



.1 siniciuri ' 





r in' 



^s on P 



7i^^ BEGINNINGS OF LIFE. 



573 



can possibly survive, and as, consequently, there is a fre- 

 quently recurring struggle for existence, it follow? that 

 any being, if it vary, however slightly, in any manner 

 profitable to itself, under the complex and sometimes 

 varying conditions of life, will have a better chance of 

 surviving, and thus be naturally selected. From the 

 strong principle of inheritance, any selected variety 

 will tend to propagate its new and modified form. . . . 

 Thus the small differences distinguishing varieties of 



r 



the same species steadily tend to increase^ till they 

 come to equal the greater differences between species 

 of the same genus^ or even of distinct genera/ 



mod 



come 



into play until homogenesis becomes the rule, and the 

 continued influence of heredity is rendered possible. 



Natural 



t> 



the modifications of that vast assemblage of animal and 



timeSj that no 



And this beiogti; vegetal forms which we have proposed to include under 



the name of ^ ephemeromorphs/ It must, indeed, be 

 hmited to *^ species/ such as we now define themj and 

 we propose very briefly to consider to what extent such 

 agencies operate amongst these forms as producers of 

 variation, and to what extent they are supplemented 

 by the action of other modifying influences. 



It was pointed out 



Mr. Herbert Spencer^ in 



1864 that a certain amount of ambiguity attached to 



' Principles of Biology/ vol. i. p. 445. 



\ 



