DARWINISM CHAP. 



animals and plants which live together in any one country, 

 and to give some rational account of the phenomena presented 

 by their distribution in different parts of the world. And, 

 lastly, we may expect it to explain many difficulties and to 

 harmonise many incongruities in the excessively complex 

 affinities and relations of living things. All this the Darwinian 

 theory undoubtedly does. It shows us how, by means of 

 some of the most universal and ever-acting laws in nature, 

 new species are necessarily produced, while the old species 

 become extinct ; and it enables us to understand how the 

 continuous action of these laws during the long periods ^Yiih 

 which geology makes us accpxainted is calculated to bring 

 about those greater differences presented by the distinct 

 genera, families, and orders into which all living things are 

 classified by naturalists. The differences which these present 

 are all of the same nature as those presented by the species of 

 many large genera, but much greater in amaunt ; and they can 

 all be explained by the action of the same general laws and 

 by the extinction of a larger or smaller number of interm-ediate 

 species. Whether the distinctions between the higher gi'oups 

 termed Classes and Sub-kingdoms may be accounted for in 

 the same way is a much more difficult question. The differ- 

 ences Avhich separate the mammals, birds, reptiles, and fishes 

 from each other, though vast, yet seem of the same nature as 

 those which distinguish a mouse from an elephant or a 

 swallow from a goose. But the vertebrate animals, the 

 mollusca, and the insects, are so radically distinct in their 

 Avhole organisation and in the very plan of their structure, 

 that ol)jectors may not unreasonably doubt whether they can 

 all have been derived from a common ancestor by means of 

 the very same laws as have sufficed for the differentiation 

 of the various species of birds or of reptiles. 



The Change of Opinion effected hy Darwin. 



The point I A\dsh especially to urge is this. Before 

 Darwin's work appeared, the gTeat majority of naturalists, and 

 almost Avithout exception the Avhole literary and scientific 

 Avorld, held firmly to the belief that species Avere realities, and 

 had not been derived from other species by any process 

 accessible to us ; the different species of croAv and of Aaolet 



