96 MAMMALIAN DENTITION 



so thoroughly specialized that only obscure traces remain to 

 show what from the paleontological record we know its ances- 

 try to have been but it is certain that all orders at one time or 

 another, some in the very remote past, have originated from an 

 insectivorous-omnivorous type. 



The foregoing statement will render clear the reason for 

 studying the Insectivores first among placental orders. Many 

 years ago Huxley laid particular stress upon this central posi- 

 tion of the Insectivora and announced his conviction that any 

 one who is acquainted with the range of variation of structure 

 in this order and in the Rodentia possesses the key to every 

 peculiarity met with in the Primates, Carnivora and Ungulata. 

 How true Huxley's statement has proved to be will become 

 apparent in the perusal of the ensuing chapters. 



The order Insectivora is without doubt the most ancient 

 placental order and the animals included therein subsist upon 

 an insectivorous diet with its usual adjuncts, — worms, rep- 

 tiles, eggs, nestlings and fruit except in the case of the Afri- 

 can Water Shrew Potamogale which is said to feed entirely 

 upon fish. Animals which have preserved to the present time a 

 phylogenetic character so ancient cannot be expected to 

 exhibit the generalized features of their ancestors. Though 

 certainly primitive all are exceedingly specialized and aber- 

 rant. They are sparsely distributed in the more inaccessible 

 parts of the world or have developed some special habit of 

 life (Moles) or some particular method of defense (Hedgehogs) 

 whereby they are enabled better to preserve their existence. 



Of the Insectivores there are two types distinct in their den- 

 tition though similar in their diet.* Of these the Dilambdo- 

 donts present W-shaped upper molars like those of the insec- 

 tivorous Marsupialia, whereas the Zalambdodonts exhibit a pe- 

 culiar triangular type of upper molar which has probably been 



*This seems in direct contradiction of the general statement so often emphasized 

 in these pages that like diets are associated with similar types of dentition even in 

 different orders. In view of our ignorance concerning the mode and cause of evolu- 

 tion of the Zalambdodont dentition, one can only say that this, the most striking excep- 

 tion cannot weaken the case for the relationship in general of dentition with die* 

 which stands self-evident (but see pp. 197, 223). 



