io ARBOREAL MAN 



which its limbs enable it to walk at the bottom of its 

 tank, to clamber over obstacles, or climb aquatic plants. 

 But we note that when on dry land its activities are con- 

 siderably hampered, since while its limbs propel it for- 

 wards they no longer carry its weight, and the body 

 dragged along the ground. " On thy belly shalt thou 

 go " applies to the pioneers of the land-living Vertebrates ; 

 for their limbs are not yet adapted to supporting their 

 bodies and carrying them sheer of the ground. 



Mobility is the keynote of this primitive limb. WitJ 

 the permanent exchange of an aquatic for a terrestrial 

 habitat the limbs took on a new function, for in addition 

 to acting as mere propellers, they now serve to lift the 

 body during the act of propulsion. With this change a 

 new demand is made in the structure of the limb, for 

 stability must be added to mobility. There is a gradual 

 evolution of this new function. The limbs at first support 

 the body only during the act of propulsion; when the 

 movement is over, the body sinks to rest upon the ground. 

 In the next phase the support of the body by the limbs 

 becomes permanent ; the demand for stability in the limbs 

 is increased. 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 developmental needs that the great 

 interest of the study lies. 



In such a question as this the records of paleontology 

 are likely to furnish much material assistance, and it is 

 from the paleontologist that the most definite pronounce- 

 ments may be expected. The remains of animals furnish 

 some clear guides as to the possibility of their limbs being 

 supporting as well as propelling organs, and the geological 

 period at which animals possessing such limbs first 

 appeared seems to be generally agreed on. We find in 

 this feature, as we shall repeatedly find again in relation 

 to other things, that the search for these animals must be 



