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 

 mieration is a necessarv condition for natural selection," as a 



Oft/ ■' 



special " law of onigration " ; but we consider it sufficiently- 

 refuted by the above-mentioned facts. We have, moreover, 

 ah-eady pointed out that in reality the origin of new species 

 by natural selection is a matheTYiatical 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 clistrihution — 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 iLt 



