292 



SCIENCE. 



[N. S. Vol. V. No. 112. 



tained deciduous tooth) is nearei' the canine 

 than in any othei- variety. The interval 

 between the third and fourth upper premo- 

 lars is conspicuous in the rather short-faced 

 Esquimaux dog and the St. Bernard. 



Since in the domestic dog the increase in 

 the length of the lower jaw occurs in phyl- 

 logeny (artificially in breeding) and in 

 ontogeny the form is of exceptional value in 

 the philosophic study of senility and dis- 

 eased action. But the subject is corre- 

 spondingly complex. So far as I know, no 

 collections are available for its satisfactory 

 study. 



In old examples of Glossophaga soricina and 

 Pteropus poliocephalus the intervals between 

 the premolars is much greater than in 

 younger individuals. 



V. Senile forms of one speeies may resemble in 

 essential characters the typical forms of an allied 



The appearances forming the basis of this 

 proposition may be found not only in aged 

 animals, but in the vigorous stages of an al- 

 lied species as well. Thus, an old example 

 of G. soricina resembles the typical form of 

 a closely allied species, G. truei. * 



VI. Gross variations in the forms of teeth in 

 closely related and highly specialized animals in- 

 dicate that the types have become exhausted of 

 their capacity to precise adaptation and are de- 

 generating. The forms that degenerated teeth 

 assume are those that simidate senile changes in 

 animals less highly specialized than themselves. 



One of the most familiar changes incident 

 to long use is the wear of the teeth. lb 

 would appear that the height of the tooth 

 is in some degree proportional to the work 

 that is expected of it ; that the crown repre- 

 sents the accumulation of a definite amount 

 of material to be used up in grinding. 

 We have often occasion to note the effects, 

 upon the jaws themselves, of the lowering of 

 the height of the crowns of the teeth. 



The fox bats, which are frugivorous, are 

 * Proo. U. S. National Museum, 1896, 779. 



composed of fifteen genera. With a single 

 exception {Pteralopex) , they retain molars 

 whose cusps have almost disappeared. The 

 fact can be otherwise expressed as follows : 

 The departure from the habit of the con- 

 sumption of animal food to that of fruit leads 

 to rather abrupt changes in the tooth form, 

 by which the cusps are rapidly worn away, 

 one insular genus alone maintaining its an- 

 cestral cuspidation. 



. In the New World fruit-eating bats we have 

 examples of similar abrupt changes, though 

 carried out in a less uniform manner. The 

 forms do not constitute in themselves a fam- 

 ily, but are grouped irregularly within the 

 family, most of the members of which re- 

 tain carnivorous habits, and at best the 

 lines between the carnivorous and the fru- 

 givorous types are not sharply defined. In 

 the Vampyri out of thirteen genera the 

 molars of one genus only (Hemiderma) have 

 lost their cusps, while in the Glossophagina 

 out of six genera the teeth of one doubtful 

 member of the group (Phyllonycteris) have 

 lost their cusps. But in the Stenodermina, 

 of thirteen genera, all the cusps have been 

 lost, or are retained in the shape of the 

 merest rudiments of the carnivorous plan. 

 In two remote genera ( Cephalotes and Ecto- 

 phylla) , one in the Old World and the other 

 in the ]!^ew, the concave depressed molar 

 crown has been so worn away as to bring 

 the grinding surface of the enamel cap near 

 the alveolus and apparently to create a new 

 system of cusps, as if the tooth were a slate 

 on which had been placed a problem which 

 demanded tliat an older problem previously 

 recorded on the same surface should be re- 

 moved before the terms of the newer one 

 could be stated. In the senile form of 

 every bunodont mammal the molar teeth 

 tend to have the crowns diminish by wear 

 and all traces of cuspidation to be lost. To 

 my mind there is no difference between 

 the loss of all cusps in the last premolar 

 and the molai-s of an old dog, and the way 



