CLASSIFICATION. 67 



Within one phylum a form of animal organisation is evolved 

 along the most varied lines, which gradually lead from the simple to 

 the more complex, and from the lower to the higher. The categories 

 which we distinguish as species, genera, families, orders, and 

 classes are due to continued differentiation. These subdivisions 

 correspond to the ramifications of the branch, and in them the 

 divergence of character is expressed. 



The differences between the classes, orders, and so on, are so 

 great that they do not seem to have any connecting links at all, but 

 we must take into consideration the fact that in living forms we 

 have before us only the final offshoots of developmental series of 

 organisms, which have been ramified in very various ways, and 

 which lived in earlier and often very far-distant periods, and which 

 have gradually disappeared. The paheontological record proves 

 this partly, though it may be but very slightly. In the strata of the 

 earth remnants of forms which have disappeared, and which were 

 the predecessors, and, in fact, the direct ancestors, of later living 

 organisms, are preserved. Inasmuch as the living forms are but a 

 small portion of the whole world of organisms, which has existed in 

 the course of geological periods of development, we cannot expect 

 that far-distant connections should be always evident, the inter- 

 mediate steps determinable, and the genealogical connection made 

 clear and indubitable. It is necessary to try and put the whole 

 together out of fragments and to find lost traces of continuity. 

 The most important part of the business of Comparative Anatomy is 

 to find indications of genetic connection in the organisation of the 

 Animal body. 



Following out this conception we have to represent to ourselves 

 a developmental series of organisms arising in each phylum from a 

 primitive form, which has been, during geological development, 

 differentiated into many branches and twigs, most of which have 

 disappeared at different periods, while some, greatly changed 

 though they may have been, have lived on until to-day. The 

 general character which has remained in these various stages of 

 differentiation, and has been transmitted, with modifications, from 

 the stem-form, is what is typical in the organisation. 



§ 57. 



It is not always possible to prove, to the same extent, in all of 

 the large divisions which are regarded as types, the common ancestry 

 of the forms which belong to it. It is in fact very probable that 

 several divisions have had. a polyphyletic origin, in which case the 

 organisms which belong to them must be united together for reasons 

 other than genealogical. Such divisions cannot be regarded as phyla. 



We meet with such relations in the lowest forms, in the 

 boundary-territory between Animals and Plants. It is difficult to 

 find a boundary line, for there are organisms which seem to belong to 

 one as much as to the other Kingdom according to the phenomena 



f 2 



