3588 The Zoologist — July, 1873. 



There is a solution which may perhaps be offered, that the 

 higher rodents and Carnivora arose on parallel lines from the 

 marsupial rodents and Carnivora ; but in that case it will give 

 the same diflSculty in another form, for it will admit that the 

 placental type had arisen from at least two separate origins, which, 

 according to our previous argument, is infinitely improbable. 



The more I ponder the subject the more I am convinced that 

 the difficulty is no mere quibble. To look at the three skulls, of a 

 Thylacinus, a dog and a seal, and to consider that by any possible 

 genealogy the dog is more nearly related to the seal than to the 

 Thylacinus, and in fact that before the relationship between the 

 dog and marsupial can be traced every sign of a carnivorous 

 animal must have been lost and reproduced, presses it strongly 

 upon my mind that there is some force at work unaccounted for 

 by the theory of the evolution of species in their struggle for 

 existence. 



Let us review the complexity and apparent difficulty of the 

 evolution of teeth in such definite form and arrangement as those 

 I have been describing. I say apparent difficulty, because in 

 making researches the student can hardly fail to be impressed with 

 a feeling as if ages upon ages had been spent, and myriads of forms 

 evolved for every little step in advance. 



I will try and give a general outline of what seems to have been 

 the path of the evolution of teeth, as a great deal of the strength of 

 my argument is based upon the very high type of organization 

 which they evince. 



We do not find that teeth maintain any important place in the 

 animal economy until we arrive at the subkingdom Vertebrata. 

 There are a few curious examples among the lower forms, as in 

 Echinus, the leech, and amongst mollusks ; but it is amongst 

 animals possessing a bony skeleton that teeth are met with in 

 endless variety of form, structure and arrangement. Amongst the 

 lowest vertebrates (fishes), we find, as we should expect, the lowest 

 types of teeth, some of which seem to consist of a tissue scarcely 

 varying from bone in structure, so that it may be well to say a ievr 

 words about bone itself as illustrative of our subject. 



The essential requisites of bone seem to be, that it shall possess 

 a certain amount of mechanical strength and hardness, in order to 

 support the soft parts and provide them with rigid bars to be used 

 as motile levers; also that it shall be capable of such change of 



