DIFPERENOES IN HISTOLOGICAL STKUOTURE OF TEETH. 467 



great reduction iu their teeth, and that a more powerful den- 

 tition had again been evolved, without any recovery of the 

 minute structure which belongs to the larger teeth of those 

 which have never lost strong dentitions. The teeth round the 

 margins of the mouth in Uraleptus are villiform, with large ones 

 interspersed. However, these and similar speculations may be 

 more safely indulged in by those who have a more intimate 

 acquaintance with these fishes than I can pretend to, and it 

 will suffice for me to merely point out the facts. 



It may, perhaps, seem that in this communication a very 

 small point has been laboured, but it appears to me that such 

 facts as these are worthy of being recorded, for, so far as I 

 know, very little has been done in the way of investigating the 

 existence or non-existence of structural change in tissues under 

 the influence of evolution. And the existence of these struc- 

 tural differences seems to me to have some bearings of a wide 

 nature. 



The teeth of the Gadida? appear to furnish an argument 

 against the adequacy of the purely mechanical theory of the 

 evolution of tooth forms, so warmly advocated by Cope under the 

 name of kinetogenesis, and adopted in its entirety by a large 

 number of the American school of naturalists. For if the form 

 of a tooth is the direct consequence of the direction of stimu- 

 lation that it has received by use in successive generations, 

 then a tooth which is subject to the very minimum of use, such 

 as that of the gill-rakers of the hake, ought not to be so exact 

 a copy of the teeth round the margins of the mouth. And if 

 very considerable use be essential to the maintenance of elabo- 

 rate structure, then we might expect, on the one hand, that 

 the teeth in the gill-rakers of the hake should be of very 

 simple structure, which is not the case, and, on the other hand, 

 that the large teeth of Uraleptus, which must be held to be 

 important in function, and so to receive the stimulus of use, 

 should not have lost the structure typical for the family whilst 

 retaining the size, and more, indeed, than the average size. 

 Hence mechanical theories do not suffice to account for 

 the structural degradation. For such it must be termed 



