THE EVOLUTION OF TYPES 269 



cestral forms, nevertheless the abrupt change must have be- 

 come stereotyped very early since Ave find it present in the 

 Cynodonts.* The relation of teeth to jaws, to musculature and 

 to the temporo-mandibular joint must be regarded as one fac- 

 tor only of those responsible for bringing about existing modi- 

 fications of the several tooth forms. The very fact that we see 

 in all orders of Mammals a definite tendency on the part of the 

 dentition to evolve along one or another of certain well-defined 

 lines itself suggests an internal heritable factor. 



The teeth of the modern Tapir are practically indistinguish- 

 able from those of its Miocene forerunner yet we cannot say 

 why this animal has undergone no change during so vast a 

 period in which other animals, the Horse and the Whales for 

 example, have developed striking dental modifications. 



We have dealt fairly comprehensively with the varied forms 

 of teeth represented today in the single order Marsupialia and 

 have noted that some members retain quite primitive forms 

 of teeth although in others the dentition is highly specialized. 

 Emphasis has been laid upon food habits as a factor responsible 

 for these differences but food habits alone have not produced 

 the change. Had we taken any series, the Bandicoot family 

 for example, and studied the varying forms of teeth exhibited 

 by these animals whose food habits differ little if at all, we 

 should still have noted progressive modifications which in the 

 Bandicoots culminate in the most specialized form Thalacomys. 



Instead of dilating upon this theme at wearisome length in 

 the chapter on the Marsupials a further consideration of these 

 progressive changes of less degree has been postponed until the 

 student should be better acquainted with the end results, as it 

 were, of Nature's experiments. We are now in a position to 

 appreciate the importance of these gradually progressive 

 changes. We might lay out in order of succession a series of 

 examples of any family or even genus taken from different 

 geological epochs and, comparing them, note the ever changing 



*The abrupt change from simple incisiforrh to crushing molariform teeth is to be 

 found even in certain fishes. 



