158 TIIE DOCTRINE OF DESCENT. 
the doctrine of Derivation seemed certain to the greatest 
minds, on philosophical and @ griori 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, 
