RECENT AND FOSSIL SPECIES. 373 
We are therefore of the same opinion as Darwin and 
Wallace, that the migration of organisms and their isolation 
in their new home is a very advantageous condition for the 
origin of new species; but we cannot admit, as Wagner 
asserts, that it is a necessary condition, and that without it 
no species can arise. Wagner sets up this opinion, “ that 
migration is a necessary condition for natural selection,” as a 
special “law of migration” ; but we consider it sufficiently 
refuted by the above-mentioned facts. We have, moreover, 
already pointed out that in reality the origin of new species 
by natural selection is a mathematical and logical necessity 
which, without anything else, follows from the simple com- 
bination of three great facts. These three fundamental 
facts are—the Struggle for Life, the Adaptability, and the 
Hereditivity of organisms. 
We cannot here enter into detail concerning the numerous 
interesting phenomena furnished by the geographical and 
topographical distribution of organic species, which are all 
wonderfully explained by the theory of selection and 
migration. For these I refer to the writings of Darwin,! 
Wallace, *® and Moritz Wagner,“ in which the im- 
portant doctrine of the limits of distribution—seas, rivers, 
and mountains—is excellently discussed and illustrated by 
numerous examples. Only three other phenomena must 
be mentioned here on account of their special importance. 
First, the close relation of forms, that is, the striking “family - 
likeness ” existing between the characteristic local forms of 
every part of the globe, and their extinct fossil ancestors in 
the same part of the globe; secondly, the no less striking 
“family likeness” between the inhabitants of island groups 
and those of the neighbouring continent from which the 
