ORIGIN OF THE MAMMALIA 
followed, of course, by advancing ability and developing brain. 
At any rate before long — geologically speaking — we find that 
the hairy mammals became numerous, and that the reptiles 
correspondingly decreased, till finally the dinosaurs, pterodac- 
tyls, and other cool-blooded, sluggish, massive land reptiles had 
died out, and the earth was ruled by warm-blooded, active 
mammals and birds. 
The traceable history of mammals extends back to the Tri- 
assic, the oldest of the three divisions of the Mesozoic era, but 
only three or four Triassic specimens have been dis- hee 
Covered, and these are so imperfectly preserved that ™orpha. 
they give little information. Just preceding that time there 
flourished a group of reptiles, the Theromorpha, which were 
large terrestrial forms with skull, teeth, and fore limbs surpris- 
ingly like those of mammals, and which fill an intermediate 
place between the highest amphibians of that time and the low- 
est of the mammals; “and it is altogether probable that from 
amongst some of their number .. . the Mammalia arose.” 
But this by no means clears up the problem, for the solution 
of which future information must be awaited. 
It is not doubted, however, that true mammals, — though 
few and very small and inconspicuous, as they would need to 
be in a land filled with ravenous reptiles, — existed throughout 
the whole Mesozoic era; and before its close the two grand 
divisions of Mammalia, Prototheria and Eutheria, had become 
established; and the two primary divisions of the latter, the 
Marsupials and Placentals, had already been separated. Then 
came the time when the mammals began to go ahead and pos- 
sess the earth, — a process pictured by Prof. W. B. Scott: — 
“The passage from the Mesozoic to the Tertiary was marked by wide- 
spread and very important changes in the physical geography of the north- 
ern hemisphere and by an extraordinary change in the life of the earth. 
Vegetation had attained almost its modern state. . . . The huge and bizarre 
reptiles of the Mesozoic had all disappeared, while the mammals came to 
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