15 



living species, but also the fossils of one area are different 

 from those of another, where there has been or is a barrier 

 to their free migration. The species which inhabit opposite 

 sides of lofty mountain chains, or of great rivers or of 

 deserts, differ remarkably ; and this is much more the case 

 with the inhabitants of widely-separated continents. The 

 marine flora and faima of sea-shores separated by deep and 

 extensive seas differ in an equally marked manner ; while 

 continents, such as Europe, possessing few barriers to free 

 migration, have a striking conmiunity of living productions. 

 The case is still more suggestive when we consider the 

 life-forms of oceanic islands. The more isolated the island 

 the fewer is the number of species to be found on it, the 

 greater is the absence of animals and plants, now repre- 

 sented on neighbouring continents, and especially of such 

 animals and plants as are incapable of migration ; and the 

 more remarkable is the similarity between the extant and 

 extinct organisms of such islands. 



Moreover, the genera of an oceanic island are often 

 peculiar to it, and the nearest allies of such living genera 

 are often to be found in the fossil remains of the nearest 

 continent. Where there is a great number of other islands, 

 between an oceanic island and the mainland, which may 

 serve as halting-places, the differences betvreen the organic 

 life of the remote island and of the continent are less 

 marked than they are in the solitary islands of a great 

 ocean. This is exactly what one would expect to find, for 

 the essential relative positions of the great continents and 

 the great deep seas have, amid minor changes, probably 

 remained fairly constant throughout geological times. 

 Consequently the only colonists of oceanic islands would 

 be such animals and plants as might be floated by trees, 

 or carried by winds, or endowed with sufiicient vitality to 

 resist the injurious influence of sea-water while conveyed 

 in the strong currents of the ocean. 



No other hypothesis than that of a common descent will 

 account for the facts of the geographical distribution of 

 living forms, while that hypothesis is perfectly sufficient to 

 explain these facts, even when they are apparently most 

 incongruous. 



The Argument from artificial Selection or Breeding. — What 

 nature does on her stupendous scale man also ett'ects in a 

 minor degree ; and in this, as in so many other branches of 

 science, the adaptations of man suggest an explanation of the 



