Chap. VII.] THEORY OF NATUEAL SELECTION. 313 



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 

 domestic races, and which through the aid of natural 

 selection would equally well give rise by graduated steps 

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

 rally 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 would extend 

 the same view to the wings of bats and pterodactyles. 

 This conclusion, which implies great breaks or discon- 

 tinuity in the series, appears to me improbable in the 

 highest degree. 



Every one who believes in slow and gradual evolution, 

 will of course admit that specific changes may have been 

 as abrupt and as great as any single variation which we 

 meet with under nature, or even under domestication. 

 But as species are more variable when domesticated or 

 cultivated than under their natural conditions, it is riot 



