258 Likenesses; or, Philosophical Anatomy 



attempts to explain these and all facts of structure, not due 

 to inheritance, by the action upon each organism of its 

 environment. Thus he explains the very general absence 

 of symmetry between the dorsal and ventral (upper and 

 lower) surfaces of most animals by the different conditions 

 to which these two surfaces are respectively exposed. But 

 it may be objected that this is no real explanation, but a 

 mere restatement of the facts. No reasons have been given 

 by him showing either how or why each organism so re- 

 sponds to such external differences of environment, or how 

 such differences of environment tend to produce such par- 

 ticular modifications. Mr. Spencer, indeed, beautifully illus- 

 trates that correlation which, however produced, all must 

 admit to exist between the structure of organisms and their 

 surrounding conditions, but he quite fails to show that such 

 conditions are the cause of such structure. His argument is, 

 indeed, an example of the old fallacy, "post hoc ergo propter 

 hoc. We believe the cause to be, not external, but internal. 

 If animals and plants respond so readily to the action of 

 external incident forces, it must be the case that conditions 

 exist in such animals and plants which dispose and enable 

 them so to respond, according to the maxim, Quicquid 

 recipitur, recipitur ad modum recipientis, as the same rays 

 of light which bleach a piece of silk blacken nitrate of silver. 

 If, therefore, we attribute the external forms of organisms to 

 the action of external conditions, we but remove the diffi- 

 culty a step back, since we must conceive an internal power 

 and tendency occasioning such ready modifiability of struc- 

 ture. But, indeed, it is not at all easy to see how the 

 influence of the surface of the ground, or any conceivable 

 similar external condition or influence, can produce such 

 diflerences as those existing between the dorsal and ventral 

 shields of the carapace of a tortoise. 



The likenesses, then, which exist between arm and leg, 



