i8o Darwin, and after Darwin. 



colour, which cannot be conceived as serving any 

 other purpose, may enable the sexes of the same 

 species quickly to recognize each other, is not here 

 available. The species of the genus Macropus are 

 more conspicuously distinguished by differences of size 

 and form than by these minute differences of colour ; 

 and therefore no such use can be attributed to the 

 latter. And, as previously stated, even within the 

 order Marsupialia the genus Macropus is not at all 

 exceptional in this respect ; so that by including 

 other genera of the order it would be easy to gather 

 such apparently indifferent specific characters by 

 the hundred, without any one of them presenting 

 evidence — or even suggestion — of utility. How robust 

 therefore is the faith of an a priori conviction which 

 can stand against such facts as these ! What, then, 

 are the a priori grounds on which it stands? 

 Mr. Wallace, the great leader of this school of thought, 

 says : — 



" It is a necessary deduction from the theory of natural selec- 

 tion, that none of the definite facts of organic nature, no special 

 organ, no characteristic form or marking, no peculiarities of 

 instinct or of habit, no relations between species or between 

 groups of species, can exist, but which must now be, or once 

 have been, useful to the individuals or the races which possess 

 them '." 



Here, then, we have in brief compass the whole 

 essence of our opponents' argument. It is confessedly 

 an argument a priori, a deduction from the theory 

 of natural selection, a supposed consequence of that 

 theory which is alleged to be so necessary that to 



' Contributions to the Theory of Natural Selection, p. 47 (1870) ; re- 

 published in 1892. 



