158 THE DOCTRINE OF DESCENT. 



the doctrine of Derivation 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 in- 

 dubitably the most important, if not the all-sufficing 

 cause of transition is everywhere effective, and in many 

 cases sufficient." 



We wished to set forth these words of a talented philo- 

 sopher for the benefit of those who are so unreasoning 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 stationary 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, 



