34 THE MAMMALIA. 



allied with those of the present reptiles (e.g. the 

 allantois). The earliest traces of mammals from 

 the Triassic rocks lead us to suppose a long series 

 of predecessors, and direct our thoughts to further 

 unfathomed depths of the earth's development. 



It is a different matter as regards those charac- 

 teristics which the systematic zoologist makes use 

 of, first of all in distinguishing the subdivisions of 

 the group, the differences of the instruments of 

 locomotion, more especially of the outer limbs, 

 hands and feet, and also of the dentition. The 

 function of propagation exercises universally a 

 more subordinate influence upon the outward 

 appearance and the general habitus of the animal, 

 than does nutrition. The manner in which its 

 food is acquired gives the organism its peculiar 

 stamp, apart from the outer covering that acts as 

 a protection against its enemies and climatic 

 changes, and varies accordingly; and this stamp 

 is expressed chiefly in the formation of the limbs 

 and the dentition of the mammal. Cuvier's words, 

 * Give me a tooth, and from it I will build up the 

 whole animal,' are to be taken seriously ; they 

 may be applied to almost every other individual 

 part of the skeleton, and above all to the extremi- 

 ties of the limbs. The last portion of a finger will 



