1214 Growth, its Conditions and Variations. [December, 
organization quickly made itself manifest. The reptiles sunk 
and the mammals rose to the supremacy with such seeming sud- 
-denness that it appears almost the work of miracle. Of the hard 
battle for empire which must have taken place, not a trace 
remains, We step from the Cretaceous to the Eocene era, and 
at once we pass from a world ruled by huge reptiles into one con- 
trolled by equally huge mammals. The wave of reptilian life 
rapidly declined, while that of mammalian life broke over it. The 
~ former imperial rulers of land and sea sunk into lowly creeping 
and lurking forms, while the new lords of life grew into swim- 
ming and stalking monsters of unquestioned superiority. 
It is not probable that this change took place as the conse- 
quence of an actual battle between reptiles and mammals.. More 
likely it resulted from a sharp competition for food, in which the 
mammals gained the victory, the more specialized great reptiles 
dying out through starvation, while the generalized forms de- 
creased rapidly in bulk and gained new habits. 
In regard to this superiority of mammals over reptiles one of 
its most important features was the hot-blooded organization of 
the former as compared with the cold-blooded condition of the 
latter. The result of this condition was to make the reptiles 
essentially tropical in habitat. Such forms as ventured into colder 
regions must have hibernated in the winter, and thus could not 
well have attained any extreme bulk, Hibernation was not none» 
sary to mammals. This fact at once gave them a superiority In 
colder climates, in which they could develop unopposed, and from 
which they could descend to the tropics in bulky and vigorous 
forms to compete with their reptilian predecessors for the food 
supply. 
- As for the oceanic mammals, it is not impossible that they “ 
gained their early development in colder regions, which many © 
them continue to inhabit, and thence pressed southward to sei 
pete with the great ocean reptiles. The huge toothed whales © 
the early mammalian period, typified by the gigantic Zeuglodon, 
probably were particularly well adapted to obtain food, and may 
have rapidly Swept away the food supply of their predecessors: 
In this connection there is one point of the greatest importance, 
to which we have as yet made no allusion, though it mee 
was strongly influential in causing sudden replacements of old by 
new forms of life. If the competition between old and new tyPs 
