204 THE ORIGIN OF SPECIES 



now been considered, and the others will hereafter be considered. 

 They seem to me to partake little of the character of demonstra- 

 tion, 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 article lately published in the "Medico-Chirurgical Review." 



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 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 sud- 

 denness 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 im- 

 portant kind;" and apparently he would extend the same view to 

 the wings of bats and pterodactyles. This conclusion, which im- 

 plies 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 condi- 

 tions, it is not probable that such great and abrupt variations have 

 often occurred under nature, as are known occasionally to rise 

 under domestication. Of these latter variations several may be at- 

 tributed 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, etc.; and 

 as they are widely different in character from natural species, they 



