CHAPTER VIII 



THE FATE OF THE HIXD-LIMBS 



We have hurriedly reviewed the process by which a 

 primitive Mammal with four undifferentiated and mobile 

 limbs achieved the emancipation of its fore-limb by its 

 climbing activities. It is now necessary to make an 

 attempt to follow the changes which take j)lace in the 

 hind-limb under the same circumstances. This phase is 

 rather more complex in the hind-limb, and though the 

 changes produced are not, perhaps, so great, their sequence 

 has been more liable to interruption. The most primitive 

 type of hind-limb we may imagine is an exact counter- 

 part of the picture we have drawn of the fore-limb. It 

 corresponds segment for segment, and joint for joint, with 

 those described in the fore-limb. It has all the same 

 possibilities; its fate depends in great measure upon the 

 emancipation of the fore-limb. We have pictured the 

 animal in its initial stages of tree-climbing as reaching 

 out, with its fore-limb, to obtain new holds. It is during 

 this oft-repeated interval that the fate of the hind-limb 

 is determined, for, during this interval, it becomes the 

 supporting limb upon which the body weight is thrown. 

 Here is therefore the dawn of the differentiation in 

 function of fore and hind limb ; the fore-limb is reaching 

 ahead for a new hold, the hind-limb is temporarily 

 supporting the body during the act. It must be noted, 

 hoAvever, that this supporting is of a very definite kind, 

 and is not by any means of the same nature as that which 

 is brought about in those animals which, being purely 

 terrestrial, have become typically quadruj)edal. In 



48 



