THE SPINOUS PROCESSES 107 



It would take us too far aside from our present purpose 

 to discuss the question, but we may note the observation 

 that, judging by such skeletal remains as have been 

 preserved, the Mammals, at an early period of their 

 history, were represented by an extraordinary number of 

 forms the gait of which we may presume to have been 

 but little better than a simple reptilian shuffle. The 

 active galloping and springing animals are their changed 

 and modern representatives. 



In the case of arboreal animals the problem is ap- 

 parently complicated at the outset by the fact that, 

 while some perfected tree-climbers show a highly special- 

 ized series of anteverted and retroverted spines, separated 

 by a well-marked anticlinal vertebra, others, none the 

 less well fitted for a thoroughly arboreal life, have a 

 series of uniformly directed spines, all being retroverted 

 (even if only slightly so) towards the pelvis. 



It is easy to furnish a satisfactory explanation for these 

 differences by appealing to the varied, and perfectly 

 distinctive, methods of tree-climbing adopted by different 

 arboreal animals, but it is by no means easy to determine 

 what may be the relation of these two forms to each other. 

 Either type (the divided, or the uniform spinous series) 

 might be primitive in the tree-climbers, and the one 

 might subsequently be derived from the other by altera- 

 tion of function as displayed in climbing methods. 

 Again, both types may have been definite legacies in 

 arboreal animals derived from differently constructed 

 primitive stocks; both may have been inherited types 

 with which the animals took to an arboreal life, and on 

 which they have moulded their arboreal activities. Or, 

 both of these alternative factors may be in action, when 

 we regard the whole wide range of arboreal Mammals. 

 It seems not unlikely that this last supposition is true. 



Taking a group of animals so perfectly arboreal as the 

 Edentate Sloths of South America (Brady podidce), we 

 see, combined with a very peculiar fashion of arboreal 



