" ARBOREAL MAN " 213 



i.e., normal specialisation, requires a definite symbiotic nexus 

 with the plant here again so obviously concerned, though 

 apparently merely as a mechanical aid to animal evolution. 

 The author, however, rather avoids these matters by taking 

 " adaptations," " variations," " mutations " in short change 

 for granted. He makes the following reservation, or plea of 

 ignorance : 



Change comes about in some way that is obvious ; by what channel 

 or channels it comes about concerns the present inquiry but little. How 

 it is transmitted once it came into being, how it is accumulated, perfected, 

 and handed on are questions which, despite an enormous amount of work, 

 and despite an accumulated literature of dogmatic, and sometimes 

 unjustified assertion, are at present unknown. Without touching upon these 

 problems it is proposed to examine the probable path by which the Primates 

 and Man have originated, reviewing the influences that have probably 

 reacted upon them, but leaving aside the questions as to how changes 

 have come into being, and how such changes have been inherited. 



This is at any rate getting some fundamental but inconvenient 

 difficulties of explanation out of the way. But it is scarcely 

 comprehensive treatment. 



According to Prof. Wood Jones, the ideal limb of a land- 

 living Vertebrate has to satisfy two somewhat antagonistic 

 purposes : those of stability and of mobility. 



There is an antagonism in this evolution between the advantage of 

 elaborating the ancestral, and useful, mobility of the limb, and the need 

 for the newly developed, and essential, quality of stability. It is in this 

 antagonism of development needs that the great interest of the study 

 lies. 



As an example of an ideal primitive limb, the author instances 

 that of the ordinary water newt, as we can watch it climbing 

 aquatic plants for instance. When such a creature took to a 

 terrestrial habitat, he thinks, the limbs had to do more than to 

 propel forward : they had to adapt themselves to supporting 

 the body and to carrying it sheer off the ground. The limbs 

 now had to lift the body during the act of propulsion. There is 

 thus a general evolution of " stability " over and above that of 

 " mobility." 



We should have to ask a number of questions, however, 

 with regard to the physiological requisites of such an evolution. 

 WTiat class of animals in-feeding or cross-feeding was it that 

 succeeded best in changing from an aquatic to a terrestrial 

 habitat ? What was it that appealed to the aquatic animals in 



