478 SCIENCE PROGRESS 



of structural homology deprived comparative physiology of its 

 chief aim and incitement. 



Physiology has advanced since Geoffroy wrote to a depth 

 of functional analysis of which he can have formed no con- 

 ception. It is of interest, therefore, to examine Geoffroy's 

 conclusions from the modern standpoint. Is it true in the 

 light of our present knowledge that questions of function can 

 have no voice in deciding the line which animal genealogy 

 has followed ? And, even if this prove to be so, may there 

 not be an independent science of the development of function, 

 whose value will lie in its direct contribution to the under- 

 standing of evolution ? These are questions which surely 

 demand our attention. 



The first point to be settled, in order to place on a sound 

 basis the discussion of the use of functional considerations 

 in the tracing of evolution, is the exact definition of what 

 is meant by function as used in this connection. A glance 

 at Geoffroy's classical example of likeness of structure trans- 

 cending difference of function shows us at once the shallowness 

 of his analysis. He pointed out that the fore-legs of mammals 

 and the pectoral fins of fishes correspond in structure, though 

 they perform such diverse functions as running, climbing 

 and swimming. But to us this difference seems scarcely a 

 difference of function at all. We are accustomed now to think 

 of function in connection with the individual units by which 

 it is performed — namely, the muscle cells, the nerve cells, 

 the gland cells, and so forth — which are devoted each to the 

 performance of their peculiar function. And as far as the 

 individual functioning cells are concerned, a leg used for running 

 need hardly differ from a fin used for swimming. It is possible 

 to picture the change from one to the other taking place 

 without any change whatever in the functional behaviour of 

 the cells. In each the nerve fibres might conduct the nervous 

 impulse similarly, and the muscle fibres might be excited and 

 contract in an identical manner. And, indeed, the difference 

 resolves itself, in so far as it is not simply one of structure, 

 into a different control of the organ by the central nervous 

 system. It may amount to no more than a different sequence 

 of the identical events of conduction and contraction which 

 the central nervous system determines. The first point, then, 

 to be emphasised is that no confusion must be made between 



