Chap. VII.] THEORY OF NATURAL SELECTION. 313 



the phenomena which I explain by its agency. His 

 chief arguments have now been considered, and the 

 others will hereafter be considered. They seem to me 

 to partake little of the character of demonstration, and 

 to have little weight in comparison with those in favour 

 of the power of natural selection, aided by the other 

 agencies often specified. I am bound to add, that some 

 of the facts and arguments here used by me, have been 

 advanced for the same purpose in an able article lately 

 published in the ' Medieo-Chirurgical Review.' 



At the present day almost all naturalists admit evolu- 

 tion under some form. Mr. Mivart believes that species 

 change through " an internal force or tendency," about 

 which it is not pretended that anything is known. That 

 species have a capacity for change will be admitted by 

 all evolutionists; but there is no need, as it seems to 

 me, to invoke any internal force beyond the tendency 

 to ordinary variability, which through the aid of selec- 

 tion by man has given rise to many well-adapted do- 

 mestic races, and which through the aid of natural se- 

 lection would equally well give rise by graduated steps 

 to natural races or species. The final result will gener- 

 ally have been, as already explained, an advance, but 

 in some few cases a retrogression, in organisation. 



Mr. Mivart is further inclined to believe, and some 

 naturalists agree with him, that new species manifest 

 themselves "with suddenness and by modifications ap- 

 pearing at once." For instance, he supposes that the 

 differences between the extinct three-toed Hipparion 

 and the horse arose suddenly. He thinks it difficult 

 to believe that the wing of a bird " was developed in any 

 other way than by a comparatively sudden modification 

 of a marked and important kind; " and apparently he 



