KINE TO GENESIS. 365 



and dental tissues exhibit the phenomena of nutrition 

 and waste (metabolism), common to all living organic 

 matter. Hence even the hardest osseous tissues are 

 plastic and are subject to mechanical influences to a 

 degree which is not possible to dead matter of equal 

 density. 



Fundamental differences between Vertebrata are 

 displayed by their organs of movement, but before 

 specially considering these I will refer briefly to cer- 

 tain other fundamental characters displayed by the 

 skull. In advancing from the fishes to the Mammalia 

 we observe a successive consolidation of the mandibu- 

 lar arch, and of its mode of connection with the cra- 

 nium. The mandibular arch in its entirety displays in 

 the fishes a segmented condition, generally compar- 

 able to that which characterizes the branchial arches. 

 Among Batrachians and Reptilia various degrees of 

 fixation of its suspensor (hyomandibular, quadrate) to 

 the cranium exist, and in some of them it is closely 

 united by immovable suture. The complete fusion 

 with the squamosal seen in the Mammalia is its final 

 status. The segmentation of the mandibular portion 

 of the arch seems, from the discoveries of Ameghino, 

 to have continued among some of the Lower Eocene 

 mammals, but that finally disappeared, so that in the 

 modern mammals the movable mandibular arch con- 

 sists of a single element on each side. In this history 

 we see an instance of the progressive coossification of 



parts, which results from the constant strain of use, of 



r ' ~__^ 



which many other normal and abnormal examples are 



known. This use is the act of mastication. Where 

 there is no mastication, and the jaws are used only as 

 prehensile organs, this coossification does not occur, 

 as, for instance, in the snakes. In this most special- 



/ V^ 







