XI 



by discovering in the Pampean formation great fossil animals covered 

 with armour like that on the existing armadillos ; secondly, by the 

 manner in which closely-allied animals replace one another in pro- 

 ceeding southwards over the continent ; and, thirdly, by the South 

 American character of most of the productions of the Galapagos 

 Archipelago, and, more especially, by the manner in which they differ 

 slightly on each island of the group : some of the islands appearing 

 to be very ancient in a geological sense. 



" It was evident that such facts as these, as well as many others, 

 could only be explained on the supposition that species gradually 

 become modified ; and the subject haunted me. But it was equally 

 evident that neither the action of the surrounding conditions, nor the 

 will of the organisms (especially in the case of plants) could account 

 for the innumerable cases in which organisms of every kind are beau- 

 tifully adapted to their habits of life ; for instance, a woodpecker or 

 a tree-frog to climb trees, or a seed for dispersal by hooks or plumes. 

 I had always been much struck by such adaptations, and until these 

 could be explained it seemed to me almost useless to endeavour to 

 prove by indirect evidence that species have been modified." (I, 

 p. 82.) 



The facts to which reference is here made were, without doubt, 

 eminently fitted to attract the attention of a philosophical thinker ; 

 but until the relations of the existing with the extinct species and of 

 the species of the different geographical areas with one another were 

 determined with some exactness, they afforded but an unsafe founda- 

 tion for speculation. It was not possible that this determination 

 should have been effected before the return of the " Beagle " to 

 England; and thus the date which Darwin (writing in 1837) assigns 

 to the dawn of the new light which was rising in his mind becomes 

 intelligible.* 



" In July opened first note-book on Transmutation of Species. Had 

 been greatly struck from about the month of previous March on 

 character of South American fossils and species on Galapagos 

 Archipelago. These facts (especially latter) origin of all my views." 

 (I, p. 276.) 



* I am indebted to Mr. F. Darwin for the knowledge of a letter addressed by his 

 father to Dr. Otto Zacharias in 1877, which contains the following paragraph, con- 

 firmatory of the view expressed above i " When I was on board the ' Beagle,' I 

 believed in the permanence of species, but, as far as I can remember, vague doubts 

 occasionally flitted across my mind. On my return home in the autumn of 1836 

 I immediately began to prepare my journal for publication, and then saw how many 

 facts indicated the common descent of species, so that in July, 1837, I opened a 

 note-book to record any facts which might bear on the question. But I did not 

 become convinced that species were mutable until I think two or three years had 

 elapsed." 



