1902.] NATURAL SCIENCES OF rHILADELnilA. 221 



known that the muscular system is very easily modified by change 

 of habit. In the third rank would fall the organs known as 

 nephridia or kidneys. The vascular, respiratory, locomotory, 

 special sensory, intestinal, tegumentary and skeletal organs are all 

 of less morphological value than these three for the determination 

 of far-reaching relationships. 



(d) The Jfode of Treatment of the Parts. 



The parts of the organization that should furuish the l^asis for 

 morphological study leading to classification, we have seen, are 

 those generally termed organs, and not larger complexes of these 

 organs nor yet smaller parts of them. Broadly speaking, the 

 oi-gan is an anatomical unit in the consideration of the organism. 

 And we have seen that it is necessary to fix upon certain parts for 

 primary treatment, on account of the impossibility of mentally 

 grasping the organization in its entirety. By examining in succes- 

 sion these components a synthetic concept of the whole organization 

 may be gained. And all the parts must be studied— that is, the 

 organization as a whole must be understood — before we can deter- 

 mine which of these parts are of most value for determining the 

 phyletic affinities of that particular organization. 



The mode of treatment of these organs is to compare those of 

 different organisms, comparing together only those which evi- 

 dently correspond anatomically. The more conservative organs 

 would be the basis for defining larger groups, the successively less 

 conservative for defining successively smaller groups ("small" 

 being used in the sense of " closely related"). This is a well- 

 recognized principle. 



But great divergence of opinion is found in regard to the point of 

 the mode of considering the organ, that is, which of its character- 

 istics should be employed ; and unless there be some logical basis in 

 this treatment, the classification will be inconsequent and erroneous. 



An organ may be considered with regard to any of the following- 

 characteristics : color, consistency, volume, shape, structure, 

 differentiation of its parts, position with regard to other organs, 

 and number. The guiding principles in determining which should 

 be employed as the basis of treatment are two, the degree of con- 

 servatism of the characteristic and the degree in which the charac- 

 teristic represents the organ as a whole. Judged in this way, the 



