rkable link in the world between two distinct sections of
the animal kingdom. Here is a creature half reptile, half bird;
perhaps one-third reptile and two-thirds bird. It was about the size
of the crow. A little later unmistakable bird skeletons will appear,
but still their jaws are provided with long conical teeth.

Still more interesting from our standpoint is another set of primitive
animals, utterly insignificant in appearance, but of momentous
importance on account of their later history. Among these reptiles
were a few small creatures perhaps not much bigger than mice or moles.
Their teeth were a little more complicated and specialized than the
teeth of their reptilian cousins. Between their scales were small and
sparse hairs. Almost nothing but their jaws remain to-day to tell us
anything about them. But in this humble little creature of the
Mesozoic, utterly insignificant beside the tremendous reptiles of the
time, we discern the ancestor of the mammals. These were the
progenitors of the horses and cows, of the cats and dogs, of the
monkeys and apes, of the men of to-day.

During this chalk period, which forms the last portion of the age of
reptiles, life for the first time grew to look much as it does to-day.
Now, apparently, the cold of winter and the heat of summer followed
each other in regular succession. There have been colder and warmer
periods at various times in the previous history of the earth, but
undoubtedly they were more uniformly cold or uniformly warm than now.
Ages were warm, or ages were cold, but now the earth clearly shows the
annual alternations of summer and winter, and for the first time
clearly shows the bands of climate on the earth which we know as
zones.

In the chalk period this new factor of cold works mightily in favor
of the mammals. Their reptilian ancestors were cold blooded. When the
climate was warm they were active; when the climate was cold they were
sluggish. With the continuation of the annual alternations of cold and
warm weather that had now set in upon the earth, the little birds and
mammals had in their warm blood an advantage which, in the long run,
enables them not simply to compete with their reptile forefathers, but
to outdistance them absolutely in the race. Here and there, on earth
to-day, exist a few big reptiles like the crocodiles and the boa
constrictors. But they are few and comparatively insignificant among
the multitudinous population of the globe and