XI 



ON PAIJEONTOLOGICAL EVIDENCE OF GEADUAL 

 MODIFICATION OF ANIMAL FOEMS l 



I NEED scarcely say that one of the greatest, if not absolutely 

 the greatest problem which has ever exercised the minds of 

 naturalists is that of the fixity or the mutability of species. 



Are the various specific forms under which animal and 

 vegetable life exist upon earth, now and in all times past, 

 fixed within certain narrow limits of variation, and did each 

 originally appear upon the earth without genetic connection 

 with any previously existing forms, having been created de 

 now in fact ? or have these different species been produced by 

 gradual modification from pre-existing living forms, under the 

 influence of certain laws, at present very imperfectly under- 

 stood, acting through vast and indefinite periods of time ? 



It is clear that these two views are strongly opposed to 

 each other. Both have been held and still are held by men 

 who are justly considered masters in the branch of knowledge 

 to which they relate ; and the solution of the question will 

 exercise so important an influence on the progress of zoology 

 that any real contribution towards it should be one of the 

 most welcome additions to science that a naturalist of the 

 present day can make. 



1 Lecture at the Royal Institution of Great Britain, 25th April 1873. 



This essay represents the state of our knowledge twenty-five years ago. The 

 accumulation of facts since that date, especially through the labours of the able 

 palaeontologists working in the rich fields of discovery in America has been 

 enormous. If the essay were recast now, the scheme of classification at p. 144 

 would be crowded with additional names, but the main outlines would remain 

 the same. Many gaps would be filled up, which would strengthen the argument, 

 but it is perhaps more easily followed in the comparatively simple form in which 

 it is here presented. 



