THE PHYLOGENESIS OF VERTEBRATES 35 



the manner of parasites. Many are specially adapted to the 

 capture of insects, either on or beneath the surface of the 

 ground, or on trees, and some have even developed the power 

 of flight by which they may follow their prey through the 

 air. The hosts of the vegetable feeders are as highly dif- 

 ferentiated and become specially adapted to feed either upon 

 low herbage or the leaves of trees, roots, bark or fruits, and 

 have even developed one group of oceanic forms, fitted to 

 browse upon the sea- weeds and other submerged vegetation. 



These various lines of specialization, together with the usual 

 extinction of intermediate forms, have produced a series of 

 more or less isolated groups, or Orders, the interrelationships 

 of which have been deciphered in part by the labors of anat- 

 omists, in part by those of palaeontologists, but are still more 

 or less uncertain. A suggestion of this is shown in the ac- 

 companying phylogenetic tree of mammals (Fig. 8.), which 

 takes into consideration both living and extinct groups, so 

 far as known. 



The earliest mammalian forms, of which we possess only 

 fragmentary remains, were more like the reptiles, and espe- 

 cially the theromorphs, than any now extant, but possessed 

 many of the characters of the monotremes, which may be con- 

 sidered their somewhat highly specialized descendants. To 

 this group has been given the name Pantotheria, and as the 

 ancestors of all the rest they may form the main trunk of 

 the phylogenetic tree. The monotremes are the nearest living 

 descendants, and they have been derived from them through 

 an ancient and closely related group, the Multituberculata. 

 All three of these groups were reptilian in structure, and may 

 be classed together and in contrast to all the other mammals, 

 as the Sub-class Prototheria. 



\Yhile still primitive, however, the Pantotheria began to 

 differentiate along two lines, the one somewhat resembling the 

 marsupials, the other the insectivores, and thus early these 

 two lines of development became inaugurated. Eventually the 

 reptilian characters were dropped, and the animals, passing 

 over into the Sub-class Eutheria, or typical mammals, be- 



