No. I.] OSTEOLOGY OF PCEBROTHERIUM. 3 



and need not here detain us ; the question is rather as to how 

 far homological structures may arise independently, and how far 

 they may undergo similar modifications. Clearly, until this 

 problem can be definitely settled one way or the other, the 

 difficulty of making out phylogenetic series must be very great, 

 and especially so in those cases where our information as to 

 extinct forms is incomplete, or even sometimes where the 

 entire skeletal structure is before us. A notable example of 

 such a difficulty is afforded by the extraordinary animal Chalico- 

 tJicrium, the skull and teeth of which were for many years 

 referred to the ungulates, while the feet, under the names of 

 Macrotherium, Ancylotherinm, Morotherium, etc., were ascribed 

 to the edentates, but of late the researches of Forsyth Major, 

 and Filhol have shown that they all belong to the same type. 

 The curious little artiodactyl, Leptovieryx, is known in almost 

 every detail of its skeleton, and yet its relations to the other 

 members of its suborder are very far from clear, as it has been 

 variously referred to the traguline, the deer, and the camel 

 series. This is simply because we cannot yet determine how 

 far its resemblances to other groups are due to actual relation- 

 ships, and how far to parallel development. In other families 

 of extinct mammals we are continually encountering the same 

 difficulty, and opinions on the subject of genetic connections are 

 in a constant state of flux. One school of observers tacitly 

 denies all such parallelism, and this assumption leads them to 

 regard all similarities of structure as due to phylogenetic rela- 

 tion, and this again results in the most complicated cross-con- 

 nections and reticulations instead of the ordinary view of 

 diverging lines. In some treatises it is gravely argued that 

 because a given specimen has been referred by one anatomist 

 to one group, and by another to another group, the connection 

 between the two series is thereby made at least probable. 



(3) Even more difficult to decide is the question as to how far 

 convergence of development is possible, meaning by this a sim- 

 ilar result which is reached by two or more independent lines 

 having a different starting-point. The view which is held with 

 regard to this point is to a great extent conditioned by opinions 

 as to the nature of heredity, and so some naturalists deny the 

 possibility of any real convergence, while others accept it and 

 push it to extreme limits. Mivart, for example (No. 22, p. 509), 



