NATURAL SELECTION. 9 



Instinct ; the whole question of Hybridity ; and, lastly, the 

 absence in our known geological formations of innumerable 

 links connecting all allied species. Although some of these 

 difficulties are of great weight, we shall see that many of 

 them are explicable on the theory of natural selection, and are 

 otherwise inexplicable. 



In scientific investigations it is permitted to invent any 

 hypothesis, and if it explains various large and independent 

 classes of facts it rises to the rank of a well-grounded theory. 

 The undulations of the ether and even its existence are hypo- 

 thetical, yet every one now admits the undulatory theory of 

 light. The principle of natural selection may be looked at as 

 a mere hypothesis, but rendered in some degree probable by 

 what we positively know of the variability of organic beings 

 in a state of nature, — by what we positively know of the 

 struggle for existence, and the consequent almost inevitable 

 preservation of favourable variations, — and from the analogical 

 formation of domestic races. Now this hypothesis may be 

 tested, — and this seems to me the only fair and legitimate 

 manner of considering the whole question, — by trying 

 whether it explains several large and independent classes of 

 facts; such as the geological succession of organic beings, 

 their distribution in past and present times, and their mutual 

 affinities and homologies. If the principle of natural selection 

 does explain these and other large bodies of facts, it ought to 

 be received. On the ordinary view of each species having 

 been independently created, we gain no scientific explanation 

 of any one of these facts. We can only say that it has so 

 pleased the Creator to command that the past and present 

 inhabitants of the world should appear in a certain order and 

 in certain areas ; that He has impressed on them the most 

 extraordinary resemblances, and has classed them in groups 

 subordinate to groups. But by such statements we gain no 

 new knowledge ; we do not connect together facts and laws ; 

 we explain nothing. 



It was the consideration of such large groups of facts 

 as these which first led me to take up the present subject. 

 When I visited during the voyage of H.M.S. Beagle, the 

 Galapagos Archipelago, situated in the Pacific Ocean about 



