158 THE DOCTRINE OF DESCENT. 



seemed certain to the greatest minds, on philosophical 

 and a priori grounds, there can be still less doubt as 

 to the theory of Descent, now that Darwin and 

 Wallace have plainly shown that indubitably the 

 most important, if not the all-sufificing cause of 

 transition is everywhere effective, and in many cases 

 sufficient." 



We wished to set forth these words of a talented 

 philosopher for the benefit of those who are so unreason- 

 ing as to pour away the child with the bath-water, and 

 fancy that they have slain the doctrine of Descent when 

 they have been lucky enough to raise a few cavils against 

 Darwin's theory of selection. Does the theory of selec- 

 tion fulfil every requirement? It accomplishes many 

 and great things, but in some cases it seems to be. in- 

 adequate, and in other cases it is not requisite, as the 

 solution of the formation of species is found in other 

 natural conditions. 



Moritz Wagner, a decided adherent of Metamorphosis 

 and an enthusiastic admirer of Darwin, endeavoured to 

 establish a " law of migration," namely, that " the migra- 

 tion of' organisms and the formation of colonies by them 

 is the necessary condition of natural selection." *'' In 

 his opinion, new species arise only when smaller com- 

 munities of individuals, in process of forming varieties, 

 are geographically isolated, as in this manner only is 

 intercrossing precluded with their statidnary congeners, 

 who do not participate in the transformation; and rever- 

 sion and disappearance of characters as yet not fixed 

 is thus avoided.' That isolation often acts very favour- 

 ably on the formation of species is a fact almost univer- 

 sally acknowledged and easily verified by insular fauna, 



