THE EVOLUTION OF ANIMAL FUNCTION 



PART II 



By KEITH LUCAS 



In an article published in this Journal at the beginning of the 

 present year, 1 I discussed in a preliminary way some aspects 

 of the relation between animal physiology and the hypothesis of 

 evolution. I pointed out how, in my belief, the study of animal 

 function has been prevented from developing along comparative 

 lines by a somewhat loose application of the morphological 

 doctrine of homologous parts ; and I suggested that there was 

 no valid reason known why, provided only that certain specified 

 precautions were properly observed, likeness of function should 

 not serve as a legitimate basis for tracing the course of phylo- 

 genetic development. The precautions to be observed con- 

 sisted first in the reference of function to the individual cells 

 in which it might be manifested ; and second, in a rigid dis- 

 tinction between the functional behaviour of a cell, as gauged by 

 its actual performance under the normal conditions of life 

 within the organism, and its functional capability as discovered 

 from its reaction to certain standard conditions and stimuli. 



The preliminary discussion of these necessary distinctions 

 was cast in the form of an inquiry into the use of functional 

 characters as a basis of phylogenetic research, for the reason, 

 indicated at the time, that the historical development of the 

 present position seemed to demand such treatment. But it 

 need not be inferred that the exclusion of functional considera- 

 tions from the problems of phylogeny is to be regretted only, 

 or even in the main, in the interests of phylogeny. I suggested 

 briefly at the time that the neglect of a comparative study of 

 function was to be deplored, rather because it has left us in the 

 dark about the evolution of function itself, than because phylo- 

 genetic history might have been more completely and accurately 

 traced. The statement was, however, made without more 

 discussion. I wish now to attempt to justifiy what was said 



1 Science Progress, 1909, 3. 472-83. 



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