26 GENERAL PRELIMINARY CONSIDERATIONS. 



species ; if it ever should succeed in finding a truly natural 

 system corresponding to these relations, this in itself would be 

 the best morphological evidence of the accuracy of one of the 

 principal propositions of the Darwinian theory, i.e. of the 

 genealogical relationship of all animals. But Physiology, 

 taking this result for granted as a fact, endeavours to explain 

 it by revealing its physiological necessity, i.e. its dependence 

 on external and internal causes whose united action has, 

 slowly or rapidly, caused the transmutation of one animal form 

 into another. 



It will be advisable to illustrate this proposition by shortly 

 discussing an example. Morphology teaches us that two paii-s 

 of organs of locomotion limbs are a marked characteristic of 

 the Vertebrata, and that two pairs only do not occur in any other 

 animal group. Moreover, we have learned that these two pairs 

 of extremities must have possessed the highest degree of plas- 

 ticity, since they are found, throughout the vertebrate series, of 

 the utmost variety of form and structure ; while at the same 

 time their variations are so characteristic that they furnish us 

 with an easy means of tracing even very close relations of 

 affinity between different animals. But Physiology has hitherto 

 been wholly unable to detect the causes which led to the 

 development of only two pairs of limbs in the Vertebrata ; since 

 no self-evident usefulness can be directly ascribed to the exact 

 number of four organs of locomotion, and it is undeniable that 

 many vertebrate animals could move just as well with six or 

 more legs as with four ; there ai'e fishes, too, as the Eel, which 

 are wholly devoid of them, and yet move forward with great 

 rapidity by a wriggling motion of the body ; and Snakes, which 

 have not four legs either, run, as is well known, with extreme 

 rapidity on the points of their numerous ribs. To find a reason 

 for the prevalence of four limbs in the Vertebrata, and at the 

 same time the cause of their origin, is precisely a problem for 

 Physiology. Even if an invertebrate animal were to be found, 

 which, on general grounds, might be regarded as the nearest 

 invertebrate ally of the Vertebrata, and which, moreover, in its 

 larva or embryo stage exhibited organs which in position and 

 structure might be regarded as comparable with the simplest of 



