PARALLEL DEVELOPMENT OF SPECIES. 187 



evolved ? * And, if this is possible, seeing that under apparently 

 identical physical conditions the resulting genus became extinct 

 on the American side of the Atlantic, while it still flourished on 

 the European, might it not have so happened that the genus 

 should have actually become extinct on the one continent before 

 it even came into existence on the other ? Assuming this condi- 

 tion, we should then have a true case of extinction and reappear- 

 ance. Similarly, in the case of the genus Tapirus, the tapir, it 

 is by no means certain that the eastern and western forms are not 

 the products of two distinct ancestral lines, independent in their 

 evolution of such influences as might have been brought about by 

 migration and interassociation. 65a 



It may, however, still be urged that the apparent development 

 of parallel lines on the two continents is not in reality such, and 

 that their independent convergence is merely the result of an inter- 

 mixture of Old and New World types, during a period of land 

 connection. That a union between the northern parts of the two 

 hemispheres may have existed during the Eocene, and again in the 

 Miocene and Pliocene periods, can very well be true, but that such 

 a union need not necessarily imply the absolute inter-derivation of 

 the two continental faunas can equally well be true. The complete 

 absence from one continent of some of the more abundant fossil 

 types occurring in the other, as the genera Palseotherium, Anoplo- 

 therium, and Lophiodon from America, and Oreodon from Europe, 

 proves that there must have been (granting land connection) some 

 formidable check to migration ; and this assumption is further 

 borne out by the circumstance of the irregular appearance in time, 

 on the two continents, of such animals as might readily be supposed 

 to have been able to grapple wUh a northern barrier. 



* Orohippus has been identified by many paleontologists with Hyraco- 

 therium, and Miohippus with Anchitherium. Waagen, from his studies of 

 the Indian Jurassic Ammonitidse, has arrived at the conclusion that the genus 

 Aspidoceras (Ammonites auct. pars) has descended from at least two distinct 

 generic roots, and further maintains that the same duplex or multiple origin 

 can be traced in other genera as well (" Palseontologia Indica," ser. ix., 4, 

 1875, p. 241). He also affirms that certain species of Phylloceras, as, for 

 example, P. ptychoicum and P. Benacense, common to both Europe and In- 

 dia, are the product, in the two countries, of distinct ancestral lines, con- 

 vergent modification, as " dependent on laws which were innate " in the spe- 

 cies, having brought about an identical result (loc. tit., p. 243). 



