Mammalia. 65 



discussion — unless such discussion be based upon minute histo- 

 logical, physiological, or embryonical research. Why two teeth may 

 occasionally grow where it appears to us that there should be one, is 

 as yet — and perhaps always will remain — a mystery. That the 

 phenomenon does occur we may regard as proved ; no other hypo- 

 thesis will account for the instances collected by Mr. Bateson, nor, 

 as I think, for the vagaries exhibited by skull No. 324&. Further, it 

 seems certain that in the cited instances of the occurrence of " cloven " 

 teeth, this condition is due to what I may call a process of incomplete 

 or abortive reduplication — a process of reduplication which has com- 

 menced, but never reached completion. But here again the cause is 

 at present beyond conjecture. 



As to the second point, the possible origin of the Cetacean 

 dentition by some wholesale process of reduplication, this may or 

 may not have happened. There is, I believe, no real evidence one 

 way or the other. I cannot help thinking, however, that those who 

 rack their brains for complicated theories in explanation of Cetacean 

 dentition, have overlooked the simplest explanation of all. For, if it 

 be admitted, as I think has already been suggested, by Mr. Bateson 

 — a not very difficult or unreasonable concession — that the power of 

 forming teeth is distributed along the whole length of the jaw, what 

 is more easy than the formation of many teeth in a long jaw, of 

 fewer teeth in a shorter jaw ? The exact size of such teeth, like 

 the size of an Amoeba, would be governed primarily by mechanical 

 reasons of unknown scope, secondarily by Natural Selection. Their 

 shape would fall easily under tiie influence of the latter force. (See 

 Table IV, p. ^(^i?^ 



EXPLANATION OF PLATE I. 



To Illustrate the Variation iv the cheek-teeth of the known 

 Skulls of Ommatophoca rossi. 



1 (upper) and 1' (lower), teeth of skull numbered 324a (43.11.25.4), collected by 

 Sir James Ross's Antarctic Expedition. ■ 



2 and 2' ditto of skull numbered 324i3, with same history as No: 324a. 



3 and 3' ditto of skull numbered 700, obtained by the ' Belgica.' 



4 and 4' ditto of skull numbered 897, with same history as No 700. 



5 and 5' ditto of skull numbered 4, obtained by the ' Southern Cross.' 



6 and 6' ditto of skull numbered 3, obtained by the ' Southern Cross.'' 



7 and 7' ditto of skull numbered 2, obtained by the ' Southern Cross. ' 



