86 TAXONOMY. 



eralized, and which are the more specialized members 

 of a group of organisms ? 



The structure of a highly organized animal or plant, 

 taken as a whole, is too complicated for the human 

 mind to grasp at once. Hence the suggestion to begin 

 with the study of a single organ possessed by the 

 members of the group to be classified. Study the 

 variations in form of this organ. Determine its func- 

 tion or functions. Trace out its phylogenetic develop- 

 ment, keeping constantly in mind the relation of the 

 changes in form of the organ to its function. In other 

 words, endeavor to read the action of natural selec- 

 tion upon the group of organisms as it is recorded in 

 a single organ. The data thus obtained will aid in 

 making a provisional classification of the group. 



Then another organ is selected, and its history 

 worked out in a similar way. 



The results of the two investigations are then com- 

 pared, and where they differ there is indicated the 

 need of renewed study. For if rightly understood the 

 different records of the action of natural selection 

 will not contradict each other. The investigation is 

 continued by the study of other organs, and a corre- 

 lating of the results obtained until a consistent history 

 of the group has been worked out. 



This method differs from that commonly employed 

 in being a constant effort to determine the action of 

 natural selection in the modification of the form of 

 organisms, in order to better adapt their parts to per- 

 form their function. Ordinarily little or no attention 

 is devoted to the study of the function of organs in 

 purely taxonomic works. 



