SUCCESSION OF ANIMAL LIFE 23 



destined for supremacy on land and sea, was beginning 

 its career. From small rat-like creatures the earliest 

 mammals soon rose to manifestations of size, strength, 

 and pliability which only reptiles had hitherto shown ; 

 and as the mediaeval periods of the earth's history were 

 ages of reptiles, so the prehistoric and historic periods 

 form the ages of mammals. Wave after wave of life 

 has risen from the inexhaustible depths of nature, 

 towered to a great height, and has then fallen ; yet 

 undelayed the onward movement continues. In 

 variety of life the period of mammals is the richest of 

 all. As heir of the ages, it has the offshoots of verte- 

 brates and invertebrates whose first exuberance is 

 past. Holding the promise of the future, it contains 

 the seed of the coming dominant races : and to those 

 at their prime, those which have overcome and taken 

 possession, the earth yields her heaped measure of 

 variety and abundance. 



It is therefore clear that an animal does not enter 

 upon its life unrelated in structure or habits either with 

 those about it or unadapted to the station in which 

 its lot is cast. Its body displays to the practised eye 

 unmistakable marks of its place in the organised 

 system of life ; and its structure bears witness to the 

 precise place or specific niche which it occupies in 

 that system. If a member of a dominant race, it 

 may soon step into a position which less specialised 

 animals of more lowly birth never attain. The land- 

 owner's son has the start over the labourer's. 



