368 SCOTT. [Vol. V. 



decide is the question as to how far these processes may be 

 carried. In all of the cases cited, the final result is not identity, 

 but similarity, and there is in all of them some fundamental 

 discrepancy which exposes the deceptive character of the pro- 

 cess. I have already quoted Rutimeyer's opinion as to the 

 relationship between the camels and the typical ruminants. 

 Semper found that while the shells of certain genera of land 

 mollusca exhibit a remarkable degree of convergence, the 

 structure of the soft parts was sufficient to correct the improper 

 association of these forms. Other instances might be cited to 

 show that a careful use of the comparative method is often able 

 to avoid these obstacles. On the other hand, one cannot doubt 

 that great numbers of such cases have escaped detection, as the 

 instructive instance of the Mutabilis group of ammonites 

 plainly renders probable; for if the embryological stages of these 

 shells were not preserved in the inner whorls, no palaeontologist 

 would hesitate to class them together as closely allied, or even 

 as members of the same species. In the construction of the 

 larger taxonomic groups, which embrace so many and such 

 various forms, the difficulties arising from parallelism, both 

 positive and negative, become greatly increased, and only com- 

 plete and unbroken phyletic series will enable us to overcome 

 them entirely. It is, therefore, quite impossible to determine, 

 from our present knowledge, to what extent these processes 

 may be carried, though probably few morphologists will follow 

 Mivart in the extreme position which he has taken upon this 

 question. The fixity of direction of the lines of development, 

 which is so strikingly exemplified in nearly all well-defined 

 series of descending forms, seems to offer some assurance that 

 these interrelations of various groups will not prove to be so 

 inextricably confused as to defy any attempt to unravel them. 



The supposed dual origin of the horse in the Old World and 

 the New does not offer such insuperable difficulties as one might 

 at first imagine. In both hemispheres occur almost unbroken 

 series, from the Eocene Hyracotherium to the existing Equidcz, 

 and were only one of these series known, probably no one 

 would hesitate to regard it as the true ancestral line. But 

 several of the genera, such as Hyracotherium, Pachytiolophus, 

 Hippotherium, occur both in Europe and in America, and there 

 can be little doubt that these genera originated in one continent 



