THE EVOLUTION OF ANIMAL FUNCTION 475 



doctrine it was held that in different animals there might be 

 traced parts which represented, though often with considerable 

 modification, the same constituent of the ideal plan of organi- 

 sation ; and these corresponding parts came to be called 

 homologous. 1 Working under the influence of this doctrine, 

 zoologists began to base their classification of animals on the 

 presence in them of recognisable homologous parts. It was 

 over the criteria of homology that the split between structural 

 and functional characters took place. 



Cuvier recognised this doctrine of a common plan of 

 organisation, and made use of it in working out his system 

 of classification. His method was to trace throughout the 

 animal kingdom organs which correspond in function, and to 

 find how far they were like or unlike in structure. That the 

 method was not wholly satisfactory his own words show : 

 11 On voit . . . que ce qui est commun a chaque genre d'organes, 

 considere dans tous les animaux, se reduit a tres peu de chose, 

 et qu'ils ne se ressemblent souvent que par l'effet qu'ils pro- 

 duisent." He illustrates this fact particularly from the organs 

 of respiration, which, as he points out, differ so widely in 

 different classes of animals that they often present no common 

 point of structure whatever. That the method was actually 

 wrong was first maintained by Geoffroy St. Hilaire. He 

 showed that parts of an animal which did not correspond at 

 all in function had often an underlying correspondence of 

 structure. He pointed out, for example, that the organs used 

 by vertebrates for the different functions of running, climbing 

 and swimming— namely, the fore-legs of mammals and the 

 pectoral fins of fishes — differ widely in function, but present on 

 analysis a correspondence of structure. Therefore, he argued, 

 likeness of structure is the true criterion of homology, while 

 function is an adventitious property marking merely the use to 

 which the corresponding or homologous parts of each animal 

 are applied. It would follow that as long as classification 

 proceeded under the influence of the doctrine of a common 

 plan of organisation, by making the presence of homologous 



1 I use the word " homologous " here to avoid confusion, because it is now 

 used in this sense. As a matter of fact, Geoffroy St. Hilaire, who first gave 

 a clear definition of the relation which we now term homology, used the word 

 " analogous." But analogous now means in zoological terminology like in function, 

 which is the very meaning which Geoffroy wished to exclude from his definition. 



