Organic Evolution. 201 



mating are, supposing them to be terce causes, guiding or 

 selecting agencies. Given the variations, however caused, 

 these agencies will deal with them, eliminating some, 

 selecting others, with the ultimate result that those 

 specially fitted for their place in nature will survive. 

 Neither the one nor the other deals with the origin of 

 variations. That is a wholly different matter, and con- 

 stitutes the leading biological problem of our day. Mr. 

 Wallace's suggestion is one which concerns the origin of 

 variations, and as such is worthy of careful consideration. 

 It does not touch the question of their guidance into certain 

 channels or the maintenance of specific standards. Con- 

 cerning this Mr. Wallace is silent or confesses ignorance. 

 "Why, in allied species," he says, "the development of 

 accessory plumes has taken different forms, we are unable 

 to say, except that it may be due to that individual 

 variability which has served as the starting-point for so 

 much of what seems to us strange in form or fantastic in 

 colour, both in the animal and vegetable world."* It is 

 clear, however, that "individual variability" cannot be 

 regarded as a vera causa of the maintenance of a specific 

 standard — a standard maintained in spite of variability. 



The only directive agency (apart from that of natural 

 selection) to which Mr. Wallace can point is that suggested 

 by Mr. Alfred Tylor, in an interesting, if somewhat fanciful, 

 posthumous work on " Coloration in Animals and Plants," 

 " namely, that diversified coloration follows the chief lines 

 of structure, and changes at points, such as the joints, 

 where function changes." But even if we admit that 

 coloration-bands or spots originate at such points or 

 along such lines — and the physiological rationale is not 

 altogether obvious — even if we admit that in butterflies the 

 spots and bands usually have reference to the form of the 

 wing and the arrangement of the nervures, and that in 

 highly coloured birds the crown of the head, the throat, 

 the ear-coverts, and the eyes have usually distinct tints, 

 still it can hardly be maintained that this affords us any 



* " Darwinism," p. 293. 



