THE ARBOREAL ANCP:STRY OF THE 

 MAMMALIA. 



W. D. MATTHEW. 



Within the last few years Dollo and Bensley have adduced 

 strong arguments to show that the marsupials are descended 

 from arboreal ancestors, as indicated especially by the traces in 

 modern marsupials of former opposability in the first digit of the 

 manus and pes.^ The present writer has for some time been 

 of the opinion that this is true not only of marsupials, but of 

 the placentals as well. Our present knowledge of fossil Mam- 

 malia and of the course of evolution of the various modern 

 races, enables us to foreshadow with considerable detail the 

 characters of a common ancestral group (homogeneous in 

 adaptive characters, although perhaps embracing certain differ- 

 ences in dentition etc., of very ancient origin) from which all 

 known mammals, excepting the Prototheria, are descended. 

 That there was such a group ancestral to both metatherian 

 and eutherian mammals is, I believe, reasonably certain. The 

 evidence for it is the close uniformity of these Mammalia in 

 general structure in spite of their vvnde divergence in adaptive 

 specialization, and the invariable approximation towards a cen- 

 tral type of each race whose development is known troni 

 P^ilaeontology. As a preliminary to furthci- tiisc us.Mon uc nia\ 

 point out the characters of this primitive central type. 



-SV:r :rrr small, sbill of moderate low^^ttu brain ca.so com- 

 pletely enclosed in bone, l>rain of /no-/i type compared with that of 

 rcptilia althouo-h loiocr than in the modem mammals. In ever\- 

 case where we are able to trace the descent of the large mod- 

 ern mammals, we find their direct ancestors successively smaller 

 as we pass backward in time. The horse and camel have been 

 traced back nearly to the beginning of the Tertiary ; their ear- 



