496 Causes and Course of Organic Evolution 



of the digits, and their tendency to condensation, as well as 

 the relatively more convolute brains, are all evolving features 

 along a definite line. 



We might sum up our position at this stage then by con- 

 sidering that higher urodele amijhibians, during the permian 

 period, probably evolved into purely land animals of small 

 size and promarsupial structure; that during the triassic period 

 these increased in size, in number, and in distribution, until 

 by the close of the cretaceous they had spread over an extensive 

 southern continent, and northeastward through America into 

 Europe. Their main line of advance however was into tlie 

 australian area that received many polyprotodont as well as 

 evolving diprotodont types, and after separation of that area 

 during late eocene or early miocene times they there attained 

 highest development along both lines. 



A morphological and phylogenetic question can now be 

 studied that has long appeared to the writer as one of the 

 most remarkable and suggestive in the entire range of verte- 

 brate history, though paralleled largely by the connection indi- 

 cated in tliis work between the Rotifera and several other 

 invertebrate groups. We have already shortly alluded to it, 

 and it may now be presented as follows. If the marsupials 

 are the oldest of the true mammals — and all palaeontological 

 e\idence fortifies this — , if further they gave origin to succeed- 

 ing groups — and facts seem clearly to point in this direction, 

 then in process of evolution they gave rise to two ascending 

 but divergent evolutionary lines. These are: First, a typically 

 marsupial line, which while retaining inflected lower jaw, 

 epipubic bones, double uteri and vagina, rudimentary cloaca, 

 immature foetus, etc., has still branched out in eight or ten 

 variational directions that have given us an edentate, an ungu- 

 late, a carnivorous, a creodont, a rodent, an insectivore, a der- 

 mopterous, and a lemuroid simulation in many structural 

 details and equally in environal relations. Second, an apo- 

 marsupial line also arose, which gradually departed from 

 marsupial characteristics, and assumed new peculiarities of 

 structure, such as a non-inflected lower jaw, rudimentary 



