392 ADAPTIVE VAEIATIONS. 



the plants indeed show any spines. Again spiny plants 

 are exceedingly rare in the Canaries, though much of 

 the surface, owing to long periods of drought, presents 

 the conditions which elsewhere are supposed to produce 

 spines. Though not prepared to deny that, other 

 conditions equal, aridity may favour and humidity 

 check the growth of spines, yet Wallace considers that 

 a more important condition lies in the presence or ab- 

 sence of herbivorous mammals, against whose ravages 

 the spines afford protection. Thus he mentions sev- 

 eral countries which are not particularly arid, but in 

 which spiny plants, and also these destructive mam- 

 mals, both abound. The development of the spines is 

 chiefly dependent, therefore, on the action of Natural 

 Selection, and is not a direct adaptation. In other 

 cases also Wallace believes that the " direct action of 

 the environment can have produced only a very small 

 portion of the modifications and adaptations that actu- 

 ally exist. In by far the larger number of cases no 

 such explanation is possible, and no other adequate ex- 

 planation has been suggested except variation and 

 Natural Selection." 



Though it seems to me that Wallace, by excluding 

 all other agencies, is inclined somewhat to exaggerate 

 the importance of Natural Selection, yet his explana- 

 tion of the evolution of adaptive forms seems much 

 more rational, and in much better agreement with 

 facts, than that given by Henslow. The view to which 

 the present state of our knowledge seems to me to af- 

 ford best support is one which lies more or less between 

 these two extreme explanations. It is most con- 

 veniently indicated by a diagram. Let us consider, 



