1902. J NATURAL SCIENCES OF PHILADELPHIA. 189 



the Vertebral es ! Any one cau recall the manifold opinions as to 

 the affinities of the Rotifera, of Macrobiotus and the Gordii, of the 

 Diplopode ]\Iyriopoda, of Limidus, of the Pyenogonida, and of 

 Sagitta and Pentastomum ; indeed, these examples could be much 

 increased, and we could say that only in regard to very few 

 groups is there any unanimity of opinion. Yet the phylogenist 

 may well, in his turn, note the lack of unanimity in the conclusions 

 reached in other lines of investigation. Has the cause of variation 

 been explained, or have the phenomena of cleavage and differ- 

 entiation, or those of cell division and inheritance, and how much is 

 known of the interaction of sense organ and central nervous sys- 

 tem ? The phylogenist does not answer his critics in a vindictive 

 spirit, but to show that he as well as they must reach correct 

 interpretations slowly. A view disproved is, after all, something 

 gained, for it serves to narrow the field. , The phylogenetic classifi- 

 cation of animals is perhaps the greatest task that a naturalist has 

 before him, the aim to represent the sequence and relations of all 

 known organisms, which presupposes a thorough knowledge of their 

 structure and an understanding of the phenomena of growth and 

 change. 



AVhether such a perfect classification can ever be obtained or not, 

 it is the part of the naturalist to aid in its pursuance, even if not 

 by working upon it directly. One does not stop before a task for 

 fear he cannot complete it. When one looks upon it not only as a 

 determination of relationships, but also as involving an under- 

 standing of growth phenomena, it is found to be a project demand- 

 ing the highest mental effort. 



It seems that perhaps more logical and true ideas of the relation- 

 ships of animals may be gained by a critical consideration of the 

 standpoints employed — of attempting to eliminate those that may 

 be erroneous. If the standpoints can be logically lessened, the 

 amount of the present confusion would be reduced. The animals 

 noAV living do not i*epresent more than a fragment of the forms that 

 have once existed, and very few of the latter have been preserved 

 geologically. For the vast number of forms, most of those of softer 

 structure, no paleontological remains can be expected ; and thei'e- 

 fore for the determination of the ancestry of those now existent 

 we must rely upon the study of the anatomy of the hitter. This 

 may be the anatomy of the adult stage or of the earlier stages; 



