Chap. VII.] THEORY OF NATURAL SELECTION. 201 



At the present day almost all naturalists admit evolution 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 gradu- 

 ated 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 sud- 

 denness and by modifications appearing at once." For instance, 

 lie 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 

 thau 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 cultivated than under their natural con- 

 ditions, 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 reappear 

 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, &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 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 changed 

 as abruptly as have occasionally domestic races, and for entirely 

 disbelieving that they have changed in the wonderful mannejf 



