No. 381.] A HALF-CENTURY OF EVOLUTION. 657 
more elevated and drier coast, the incoming and expansion of 
reptilian life were fostered ; with still higher plains and hills, 
besides the increasing abundance of flowers and other seed- 
bearing plants and of the insects which visit them, existence 
for birds became possible, and with them that of a few scattered 
mammals of small size and generalized structure, with similar 
insectivorous habits. 
During the age of reptiles, when they swarmed in every 
jungle, throughout the forests and over the plains, competition 
rose so high that some of them were forced to take flight, and 
bat-like, provided with membranous wings, the pterodactyls 
lived in a medium before untried by any vertebrate, and finally 
there appeared in the Ornithostoma of the Cretaceous a colossal 
flying reptile, its wings spreading twice as much as any known 
bird, with a head four feet in length, its long toothless jaws 
closing on swarms of insects or perhaps small fry of its own 
type. But the experiment, in point of numbers or capacity for 
extended flight, did not succeed. Another type assayed the 
problem with better success. There appeared feathered and 
eventually toothless vertebrates, with the fore extremities con- 
verted into pinions and the hinder ones retaining the raptorial 
reptilian form better adapted for aérial life. They eked out a 
by no means precarious existence on flying insects and seeds, 
as well as on the life in the soil or by the seaside, and rapidly 
replaced certain older reptilian types. The class of birds has 
become about four times as numerous as the reptiles, and 
outnumbers the mammals nearly six times. 
We may now review the zodlogical changes which took place 
at the time including the end of the Paleozoic and the opening 
of the Mesozoic. There was an extinction of the Tetracoralla 
and their replacement by corals with septa arranged in sixes ; 
an extinction of cystidian and blastoic crinoids, the dying out 
of old-fashioned crinoids and echinoids (Palæocrinoidea and 
Palæechinoidea), followed by the rise of their more modern 
Specialized successors. As rapidly as the brachiopods became 
diminished in numbers, their place at the sea bottom was taken 
by the more active and in some cases predatory bivalve and 
univalve mollusks. As the trilobites became extinct, their 
