840 THE ORIGIN OF SPECIES 



ered, and the others will hereafter be considered. Tliey 

 seem to me to jiartake little of the character of demon- 

 stration, and to have little weight in comparison with 

 those in favor 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 arti- 

 cle lately published in the "Medico-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 ordi- 

 nary variability, which through the aid of selection 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 generally have been, as 

 already explained, an advance, but in some few cases a 

 retrogression, in organization. 



Mr. Mivart is further inclined to believe, and some 

 naturalists agree with him, that new species manifest 

 themselves "with suddenness and by modifications appear- 

 ing at once.'' For instance, he supposes that the differ- 

 ences 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. 



