THE BEGINNINGS OF LIFE. 573 



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

 quently recurring struggle for existence, it follows 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 

 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.' 



Obviously no such cause of modification can come 

 into play until homogenesis becomes the rule, and the 

 continued influence of heredity is rendered possible. 

 So that Natural Selection can have nothing to do with 

 the modifications of that vast assemblage of animal and 

 vegetal forms which we have proposed to include under 

 the name of c ephemeromorphs.' It must, indeed, be 

 limited to c species,' such as we now define them; 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 by Mr. Herbert Spencer 1 in 

 1864 that a certain amount of ambiguity attached to 



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



