338 Zoological Society : — 



changes observed in nature, we cannot, indeed, show directly ; but 

 when we learn experimentally that, by this means, something very 

 like natural species can be produced, surely it is more accordant with 

 the sobriety of science to assume that by this means also natural 

 species have been produced, than to refer their production to another 

 principle, which cannot be shown to be in operation at all, and of 

 which, if it is in operation, we cannot show how it could bring about 

 the effects attributed to it. 



I have said " something very like natural species ; " for, as has been 

 observed above, man cannot confer upon his varieties the self-pre- 

 serving power characteristic of true species. But this is only accordant 

 with the universal analogy of the distinction between man's work 

 and the works of what we call Nature. Man always works from with- 

 out. Nature from within. But otherwise their works are subject to 

 similar conditions. The crystalline lens of the eye is formed of 

 elementary particles, held together by molecular or chemical attrac- 

 tion, as is the lens of the eyeglass. The formation of the optical 

 image, the prevention of diffraction, is brought about in each case by 

 an observance of the same principles of construction. But the eye- 

 glass is shaped and put together by a power operating from without, 

 upon masses of elementary particles, already drawn to each other by 

 their natural attractions. The lens of the eye is formed by a power 

 working from within, which draws these elementary particles together, 

 by secret processes, into positions where their natural attractions keep 

 them in the required arrangement. 



So is it, as I conceive, in the formation of species. Man and 

 Nature both bring about changes of form in organized beings by the 

 same process, namely, by directing into particular channels the ten- 

 dency to vary inherent in all organisms, " adding up " in different di- 

 rections the sum total of many changes, tending the same way. Both 

 efPect this addition by the same instrumentality, namely, by favouring 

 sexual intercourse in the organisms which show a tendency to vary 

 in the required direction, and impeding it in those which do not. 

 But man, working in this case as in every other from without, can 

 effect his " additions " only by bringing the suitable organisms to- 

 gether for the purpose of that intercourse, and keeping the unsuitable 

 apart. Nature, working, in this case as in every other, from within, 

 effects her additions by so modifying the wish for this intercourse, that 

 the animals whom she desires to bring together shall prefer each 

 other's society, and so modifying its consequences, that accidental 

 unions of organisms, whether animal or vegetable, with other than 

 the organisms suitable for her purposes, shall be incapable of seriously 

 disturbing them. To seek an explanation of the natural process in an 

 external action, seems to me as contrary to the whole analogy of our 

 knowledge, as it would be to seek an explanation of the human pro- 

 cess in an internal action. 



And yet there is an external action in nature, bearing upon the 

 constitution of species — an action admirably described by Mr. Darwin 

 under the name of the struggle for existence, and having, as I appre- 

 hend, an effect analogous to that of external action on living or- 



