258 LESSONS FKOM NA.TUEE. [CHAP. VIII. 



conditions to which these two surfaces are respectively ex 

 posed. But it may be objected that this is no real explana 

 tion, but a mere restatement of the facts. No reasons have 

 been given by him showing either how or why each organism 

 so responds to such external differences of environment, or how 

 such differences in environment tend to produce such particular 

 modifications. Mr. Spencer, indeed, beautifully illustrates 

 that correlation which, however produced, all must admit to 

 exist between the structure of organisms and their surround 

 ing 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 hoe ergo propter hoe. I 

 believe the cause to be not external but internal. If 

 animals and plants respond so readily to the action of ex 

 ternal 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, reeipitur ad inoduin 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 difficulty 

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

 tendency occasioning such ready modiQability of structure. 

 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 differences as those 

 existing between the dorsal and ventral shields of the shell 

 of a tortoise. 



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

 between hand and foot, are hardly to be explained by any mere 

 action of the environment. But serial homology is much better 

 exemplified in a very different group of animals from back 

 boned creatures namely, the group to which all insects, 

 lobsters, centipedes, leeches, and earth-worms belong the 

 group of Annulose animals. In the centipede, the body (ex 

 cept at its two ends) consists of a longitudinal series of 

 similar segments. Each segment supports a pair of limbs, 



