Evolution of Animals 497 



or no epipubic bones, single uterus and vagina, functional 

 allantoic placenta, well-formed young, etc. This line how- 

 ever split up into exactly similar forms in morphological detail 

 and en^^ronal relation to those of the marsupial line, except 

 that one of these, passing through and including the true 

 lemurs, culminated in the anthropoid apes and in man. 



Now some extremely exacting taxonomic zoologist who 

 regards the presence or absence of a rib process, a muscle, 

 a nerve, a skull bone, as a matter of fundamental importance 

 may object to these two morphological lines as involving any 

 feature of cardinal importance. But we would view the whole 

 as proof that, amongst the small primitive marsupials of tri- 

 assic age, there were focussed up all the past hereditary ten- 

 dencies of ancestral forms, as well as certain lines of easiest 

 energy flow, food distribution, respiratory repair, reproduc- 

 tive modification, that gave surprisingly similar organization 

 when varied and more or less parallel environal stimuli acted, 

 at the same time that certain wide features underwent slow 

 change, in evolution of the higher mammals; while they re- 

 mained largely unobliterated in the segregated group of the 

 australian area that includes the bulk of existing marsupials. 



With such a morphological conception to guide us, some 

 otherwise perplexing structural details are readily explained. 

 For it is extremely difficult otherwise, even by the most liberal 

 use of the homoplastic principle, to explain why the jerboa, 

 hyrax, and shrew, representing three distinct groups, have the 

 typical marsupial character of the dentinal tubules of the 

 teeth being prolonged into the enamel; and why the wom})at 

 of the marsui)ial group has evolved in this detail to the stage of 

 the groups to which the above three types belong. 



Again the inflection of the lower jaw, that is typical of mar- 

 supials but wanting in TarsipeSy is retained in Otocyon and 

 other higher mammals. The gastric glandular patch of the 

 wombat and koala is accurately continued in the beaver. I'he 

 patterns of the teeth, that are typical of the varied morpholog- 

 ical groups of the marsupials — the carnivorous, the rodent, 

 the herbivorous — are often strikingly reproduced in the eco- 

 logically corresponding groups higher. 



