258 ORIGIN OF SPECIES 



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 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 gen- 

 erally 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 nat- 

 uralists 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 appa- 

 rently 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 improb- 

 able 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 not probable that such 

 great and abrupt variations have often occurred under 

 nature, as are known occasionally to arise under domestica- 

 tion. Of these latter variations several may be attributed to 

 reversion; and the characters which thus reappear were, it 

 is probable, in many cases at first gained in a gradual man- 

 ner. A still greater number must be called monstrosities, 

 such as six-fingered men, porcupine men, Ancon sheep, Niata 

 cattle, &c. ; and as they are widely different in character from 

 natural species, they throw very little light on our subject. 

 Excluding such cases of abrupt variations, the few which re- 

 main 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 



