286 STUDIES, SCIENTIFIC AND SOCIAL chap. 



horses, whether Shetland ponies, racers, or cart-horses, 

 form one species, because they freely breed together, 

 and are known to have all descended from a common 

 stock. 



By the same test the common ass, the kiang, the qiiagga, 

 and the zebra, are each shown to be distinct species ; for 

 though sometimes two of these species will breed together, 

 they do not do so freely, they do not reproduce their like 

 but an intermediate form called a mule, and these mules 

 are not capable of reproducing their kind, as are the 

 offspring of any pairs of a single species. What Mr. 

 Darwin did was to prove, by an overwhelming array of 

 evidence and a connected chain of irresistible argument, 

 that, just as all horses and all asses have each descended 

 from a few common ancestors, so have all asses, horses, 

 quaggas, and zebras descended from a much more remote 

 common ancestral form ; and that the same thing has 

 occurred with every group of allied species. This is the 

 " origin of species " by descent with modification, or, in 

 other words, by evolution ; while " natural selection " was 

 the term applied to the set of natural causes which formed 

 the motive power and guiding principle by which the 

 change from one species to another was brought about. 



In a very few years after the publication of this theory, 

 it had literally extinguished, among all thinking men, the 

 doctrine of special creation which had before largely pre- 

 vailed ; and some, who were its most violent opponents 

 at the outset, now accept the fact of evolution as applied 

 to almost every group of organised beings. At the 

 present day there is perhaps no single naturalist of repu- 

 tation who upholds that doctrine of the independent 

 origin of each species of animal and plant, which was a 

 very few years ago either tacitly accepted or openly main- 

 tained by the great majority of naturalists. Surely no 

 such revolution in scientific thought was ever effected by 

 one man in so short a period ! 



At first the opponents of Darwinism opposed evolution 

 as well ; but of late years the opposition is directed wholly 

 to the adequacy of the causes which Mr. Darwin main- 

 tains are sufficient to explain the origin of each species 



