322 SCIENCE PROGRESS 



and to formulate somewhat more clearly the problems with 

 which comparative physiology has to deal. 



How far, then, may we expect the comparative study of 

 function to 3 r ield results of real importance to phylogeny ? 

 I venture to think that we cannot hope by this method to 

 achieve anything of the first magnitude. It is hard to believe 

 that peculiarities of functional capability will have outlasted 

 profound morphological change, so as to bridge over those 

 great gaps by which our present knowledge of phylogeny is 

 interrupted. True, we have such facts as the preponderance 

 of uric acid excretion in the reptiles and birds and its relative 

 diminution in mammals, or the presence of creatin in the 

 vertebrates and its absence from the invertebrates, pointing 

 to the belief that certain tricks of metabolism may remain 

 true over long stretches of evolution. But our knowledge of 

 phylogenetic history is already such that facts of this order may 

 be expected rather to confirm old knowledge than to make new. 

 Comparative physiology, at the best a difficult and tedious 

 method, comes too late into the field, and can hope for no large 

 share in the enterprise. 



Even if the course of phylogenetic development were wholly 

 known and catalogued, so that we could assign all known living 

 forms to their places in an historic scheme, still it would be but 

 a slight beginning made in our knowledge of the past history of 

 evolution. Though we should know the order of succession of 

 evolutionary changes, we might remain almost wholly ignorant 

 as to what the nature of those changes had been ; and for the 

 removal of that ignorance morphological observation would 

 prove, as it seems to me, of very subsidiary importance, while the 

 comparative study of function would be absolutely imperative. 



If we try to discover broadly what is the nature of the 

 differences which the course of evolution has established 

 between one organism and another, we may for the purpose 

 of analysis recognise two distinct modes of differentiation. On 

 the one hand, evolution has produced changes which have 

 affected only the functional opportunities of the cells consti- 

 tuting an organism, by changing their relation to other cells 

 within the organism. Such a change is made by the morpho- 

 logical formation of a digestive cavity, which limits the oppor- 

 tunities of the lining cells to the function of secretion or 

 absorption. On the other hand there have been direct changes 



