90 SECTIONAL ADDRESSES. 



Crushing dentitions, with the modification of skull and jaw shape and of 

 musculature which go with them, seem equally characteristic. I had 

 always believed that the horny plates and the jaws of Platypus were 

 adapted to hard food, and that that animal possessed them, whilst the 

 closely allied Echidna was toothless, because it was aquatic and lived 

 in rivers which might be expected to have a rich molluscan fauna which 

 could serve as food. But the half-dozen specimens whose stomachs 

 I have opened contained no molluscs whatsoever, and seem to have fed 

 on insect larvae, the ordinary soft bottom fauna of a stream. I do not 

 know whether this is an accideiital occurrence, dependent on a special 

 abundance of insects in the Fish river in late spring, or whether it really 

 represents the normal food. Nothing but continued observations made 

 throughout the year can justify any statements about this case. 



One of the very few animals whose food is adequately known is the 

 herring, where the long-continued researches carried out as part of the^ 

 international investigation of the North Sea have been based on the 

 examination of thousands of stomachs taken throughout the year. 



The mouth of the herring is clearly adapted to plankton, and indeed it 

 does commonly live on such a diet. But some herring may be found 

 stuffed with specimens of the bottom-living sand-eels, whilst Mr. Ford has 

 shown me others which in yoult contained nothing but cheironomus larvae. 

 Thus even here, in the case of an animal with a very characteristic type 

 of mouth, we should not be completely justified in assuming that w& 

 could predict its diet. How much less justified are we in drawing such 

 conclusions in the case of less highly modified dentitions ? 



In the face of this uncertainty can we make use of the character of the 

 dentition of fossil vertebrates for the determination of the nature of their 

 food, and thus by building up phylogenetic series investigate the gradual 

 development both of habit and their adaptation ; one without the other 

 is valueless. The classical case of the horse is, of course, familiar to 

 everyone. From the time of Huxley the story of the gradual increase in 

 depth of crown of the molar teeth and in the complexity of the pattern 

 formed by the worn edge of the enamel which coats the cusps of the molars 

 has been held to show a steady improvement in mechanism which enabled 

 the Eqmdse to take advantage of a wide extension of grass land which 

 was assumed to have occurred in Meiocene times. 



But this assumption in its ordinary form rests on the basis of an 

 inadequate analysis of all the factors involved. 



The modern horses are bigger than those of the Eocene : an ordinary 

 hackney weighs about fifty times as much as Eohippus venticolus. 



Thus, omitting from consideration the relatively greater heat loss of 

 the smaller animal, which will be of importance only in a temperate 

 climate, and also differences in basal metabolic rate resulting from other 

 effects of size, the modern horse will wear away in a day fifty times as 

 much tooth as its ancestor ; but the surface area of its cheek teeth is only 

 about fifteen times as great, so that without a deepening of the tooth crown 

 by three and a third times it would have a shorter life. 



Actually, the crown is deepened about thirteen times, so that its 

 potential longevity is increased to about four times that of Eohippus on 

 the assumption that the abrasive qualities of the food of the two animals 



