776 Comparative Morphology of Chordates 



These correspondences between marsupials and placentals are 

 many and striking. The correctness of their interpretation as results 

 of parallel (or convergent?) evolution (see p. 366) can hardly be ques- 

 tioned. It seems clear that, in animals of the same general kind (such 

 as, e.g., members of a vertebrate Class), similarity of environment, 

 diet, method of locomotion, and general mode of life will inevitably 

 be associated with similar anatomic adaptations, quite regardless of 

 close genetic relationship. This comparison of existing marsupials 

 and placentals emphasizes the danger of error in judging relationships 

 on the basis of similarity of adult structures, without adequate knowl- 

 edge of their embryonic and phylogenetic history. 



During the Eocene Epoch, the reptilian menace having become 

 much reduced, placental mammals increased and entered upon a 

 period of adaptive radiation (see p. 366). The Jurassic pantotheres did 

 not last long, but they, or as yet unknown mammals of similar primi- 

 tive nature (for the fossil material is exceedingly scanty), apparently 

 left descendants which, in size and general anatomy, were essentially 

 like modern Insectivora. The existence of Insectivora can be traced 

 from the Eocene down to the present. Early in the Eocene appeared 

 other mammals, still small and, in general, unspecialized, but showing 

 in teeth and some skeletal characteristics evidences of a tendency to- 

 ward carnivorous habits and structure — the beginning of a group called 

 "Creodonta." It was in this period, when most of the then existing 

 mammals were in that anatomically generalized and plastic state 

 which seems to be necessary for acquisition of adaptive structural 

 modifications related to changes in habitat, diet, and mode of living, 

 that placentals began to undergo the elaborate diversification which 

 produced the numerous Orders (Fig. 594). 



Evidence from fossils strongly indicates that most of the now 

 existing Orders had their beginnings in either the ancient Insectivora 

 or the contemporary incipient carnivores, the Creodonta. The main 

 line of the Insectivora has persisted to the present with only minor 

 modifications. The Chiroptera and Dermoptera are essentially in- 

 sectivores in general anatomy, but specialized for aerial locomotion — 

 actual flight in bats and sailing in the colugo. The Rodentia also 

 would seem to be closely allied to the Insectivora, but there is some 

 reason to believe that rodents go back to something even earlier than 

 Insectivora, perhaps having had independent origin from pantotheres. 



The members of the several "edentate" Orders, rather more 

 strongly than other i)lacentals, show reptilian features which mark 

 them as of ancient origin. The Xenarthra and possibly also the 

 Pholidota may be assigned to very early origin from the Insectivora 

 or from some even more primitive placentals. Some of the ancient 



