THEORY OF NATURAL SELECTION. 223 



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 in- 

 ternal force beyond the tendency to ordinary 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 natur- 

 alists agree with him, that new species manifest themselves 

 "with suddenness and by modifications appearing 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 discontinuity 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 culti- 

 vated than under their natural conditions, it is not probable 

 that such great and abrupt variations have often occurred 

 under nature, as are known occasionally to arise under 

 domestication. Of these latter variations several may be 

 attributed to reversion ; and the characters which thus re- 

 appear were, it is probable, in many cases at first gained in 

 a gradual manner. A still greater number must be called 

 monstrosities, such as six-fingered men, porcupine men, 

 Ancon sheep, Niata cattle, etc. ; and as they are widely 

 different in character from natural species, they throw veiw 

 little light on our subject. Excluding such cases of abrupt 

 variations, the few which remain would at best constitute, if 

 found in a state of -nature}' doubtful species, closely related 

 to their parental types. 



My reasons for doubting whether natural species have 



