156 MAMMALS OF THE PACIFIC WORLD 



The few fragmentary fossils known indicate that some, at least, 

 were rat-sized creatures, with pointed teeth suitable for eating 

 insects. Yet from those animals or others somewhat like them 

 began not only the genealogical lines leading to the principal 

 modern Orders, but a host of other lines which, figuratively 

 speaking, experimented with themselves for longer or shorter 

 periods of time, and, finding themselves inadequate, died out. 

 The few successful ones became the ancestors of our earliest 

 carnivores, insectivores, bats, hoofed animals, rodents, mar- 

 supials, and monotremes. Such relatively modern groups alone 

 concern us here. 



That process, involving change of shape and size combined 

 with change of use of parts, has remained continuously opera- 

 tive. The primitive five-toed horse progressively lost its outer 

 toes and became one-toed. Thus during millions of years or even 

 millions of generations a little five-toed mammal gave rise to 

 a large, one-toed horse. At the same time the remaining toes 

 increased in size and strength in order to sustain the weight 

 of the body, for horses were growing bigger. Innumerable other 

 changes in the many parts of all animals, which fitted them 

 for the performance of diverse functions, took place continu- 

 ously everywhere. This process is generally termed evolution. 



EVOLUTION OF ENVIRONMENTS 



Throughout this length of time, when the characteristics of 

 mammals were changing, alterations were taking place also in 

 the earth itself. Slow movements of the earth's crust raised or 

 lowered parts of the surface. Near the sea, uplifts pushed 

 back the ocean and caused islands long separate from the 

 mainland to become joined to it ; or downwarping permitted the 

 waters to flood the land and to convert high ground near the 

 coast into islands. On the continents long-continued earth move- 

 ments elevated mountain ranges, diverted rivers, or established 

 new rain-collecting areas. 



The slowly changing shapes of the land and oceans simulta- 



