92 Thomas Henry Huxley 



needs noue. Here we find, scattered over the globe, vegetable 

 and animal oryauisms numbering, of the one kind (according 

 to Humboldt) some 320,000 species, and of the other, some 

 2,000,000 species (see Carpenter) ; and if to these we add the 

 numbers of animal and vegetable species that have become 

 extinct, we may safely estimate the number of species that 

 have existed, and are existing, on the earth, at no less than 

 ten millions. Well, which is the most rational theory about 

 these ten millions of species ? Is it most likely that there have 

 been ten millions of special creations ; or is it most likely that 

 by continual modifications, due to change of circumstances, ten 

 millions of varieties have been produced, as varieties are being 

 produced still ? . . . Even could the supporters of the de- 

 velopment hypothesis merely shew that the origination of 

 species by the process of modification is conceivable, they 

 would be in a better position than their opponents. But they 

 can do much more than this. They can shew that the process 

 of modification has effected, and is effecting, decided changes 

 in all organisms subject to modifying influences. . . . They 

 can shew that in successive generations these changes con- 

 tinue, until ultimately the new conditions become the natural 

 ones. They can shew that in cultivated plants, domesticated 

 animals, and in the several races of men, such alterations have 

 taken place. They can show that the degrees of difference so 

 produced are often, as in dogs, greater than those on which 

 distinctions of species have been founded. They can shew, 

 too, that the changes daily taking place in ourselves — the 

 facility that attends long practice, and the loss of aptitude that 

 begins when practice ceases, — the strengthening of the passions 

 habitually gratified, and the weakening of those habitually 

 curbed, — the development of every faculty, bodily, moral, intel- 

 lectual, according to the use made of it — are all explicable on 

 this principle. And thus they can shew that throughout all 

 organic nature there is at work a modifying influence of the 

 kind they assign as the cause of these specific differences ; an 

 influence which, though slow in its action, does, in time, if the 

 circumstances demand it, produce marked changes— an influ- 

 euce which, to all appearance, would produce in the millions 

 of years, and under the great varieties of condition which geo- 

 logical records imply, any amount of change." 



