288 Bulletin American Museum of Natural History. \\o\. XXVII^ 



(2) Habits semi- arboreal, insectivorous-omnivorous. Of the existing 

 Lipotyphla none are arboreal and the majority are terrestrial, partly or 

 wholly fossorial, and often partly aquatic, and only one genus Gymnura 

 retains any distinct arboreal adaptations (p. 263), although the spreading hand 

 and foot of Ericulus may also be an arboreal heritage. Nevertheless the 

 least specialized genera of the existing Lipotyphla, namely Microgale and 

 Gymnura (Hylornys) are also those whose skeleton presents the fewest 

 differences from that of the arboreal Tupaiidae; and in the Oligocene Ictops 

 (p. 262) the skeleton, so far as known, appears less specialized in a terrestrial, 

 semi-fossorial direction than does that of its modern descendants the Erina- 

 ceidae. Of the existing Menotyphla the skeleton of the arboreal Tupaiidae 

 also appears far more generalized, especially in the limbs and backbone, 

 than does that of the terrestrial, saltatorial JNIacroscelididae. 



There is also considerable indirect evidence for ascribing to the stem 

 Insectivores semi-arboreal insectivorous-omnivorous habits. Such habits 

 seem the best fitted to give rise by adaptive radiation to all others.^ The 

 habit of running along the branches, of jumping from branch to branch, 

 favors an even development of all the muscles of the limbs, hands and feet 

 and puts a premium on a high average development of mental faculties; 

 whereas aquatic, fossorial, cursorial, saltatorial and volant habits all imply 

 limitation of movement of the limbs in particular directions, and the hyper- 

 trophy of certain parts at the expense of others, with resultant one-sided 

 specialization of the nervous system. This is fully illustrated in the various 

 types of terrestrial, fossorial and aquatic Insectivores, which are all highly 

 specialized in these particular directions; whereas the trunk and limbs of 

 the arboreal Tupaiida^ are distinguished not only by the almost entire lack 

 of hypertrophy of one part over another but also by the retention of very 

 numerous characters (such as a free centrale carpi, a third trochanter, 

 ente])icondylar foramen, pentadactyly, etc., etc.), which, on any theory, are 

 admitted to be primitive mammalian characters. 



The semi-arboreal habit also favors the retention of small size and it is 

 obvious that the opposite condition, increasing size and weight, means 

 larger muscles and greater need for the development of special processes on 

 the bones; this tends, as it were, to upset the balance of form- determining 

 forces and to start new or peculiar lines of specialization. 



The chief articles of food available to small semi-arboreal animals are 

 nestlings, eggs, insects and fruits, and this mixed fare would prevent any one 



1 Palaeontological evidence for the arboreal habit of the stem Placentals has been adduced 

 by Matthew (1904). 



