In the _Permian_ period reptiles appeared, or perhaps one should say,
began to assert themselves. That is to say, there was an emergence of
backboned animals which were free from water and relinquished the method
of breathing by gills, which Amphibians retained in their young stages
at least. The unhatched or unborn reptile breathes by means of a
vascular hood spread underneath the egg-shell and absorbing dry air from
without. It is an interesting point that this vascular hood, called the
allantois, is represented in the Amphibians by an unimportant bladder
growing out from the hind end of the food-canal. A great step in
evolution was implied in the origin of this ante-natal hood or foetal
membrane and another one--of protective significance--called the amnion,
which forms a water-bag over the delicate embryo. The step meant total
emancipation from the water and from gill-breathing, and the two
foetal membranes, the amnion and the allantois, persist not only in
all reptiles but in birds and mammals as well. These higher Vertebrates
are therefore called Amniota in contrast to the Lower Vertebrates or
Anamnia (the Amphibians, Fishes, and primitive types).
It is a suggestive fact that the embryos of all reptiles, birds, and
mammals show gill-clefts--_a tell-tale evidence of their distant aquatic
ancestry_. But these embryonic gill-clefts are not used for respiration
and show no trace of gills except in a few embryonic reptiles and birds
where their dwindled vestiges have been recently discovered. As to the
gill-clefts, they are of no use in higher Vertebrates except that the
first becomes the Eustachian tube leading from the ear-passage to the
back of the mouth. The reason why they persist when only one is of any
use, and that in a transformed guise, would be difficult to interpret
except in terms of the Evolution theory. They illustrate the lingering
influence of a long pedigree, the living hand of the past, the tendency
that individual development has to recapitulate racial evolution. In a
condensed and telescoped manner, of course, for what took the race a
million years may be recapitulated by the individual in a week!
In the Permian period the warm moist climate of most of the
Carboniferous period was replaced by severe conditions, culminating in
an Ice Age which spread from the Southern Hemisphere throughout the
world. With this was associated a waning of the Carboniferous flora, and
the appearance of a new one, consisting of ferns, conifers, ginkgos, and
cycads, which persisted until near the end of the Mesozoic era. The
Permian Ice Age lasted for millions of years, and was most severe in the
Far South. Of course, it was a very different world then, for North
Europe was joined to North America, Africa to South America, and
Australia to Asia. It was probably during the Permian Ice Age that many
of the insects divided their life-history into two main chapters--the
feeding, growing, moulting, immature, larval stages, e.g. caterpillars,
and the more ascetic, non-growing, non-moulting, winged phase, adapted
for reproduction. Between these there intervened the quiescent,
well-protected pupa stage or chrysalis, probably adapted to begin with
as a means of surviving the severe winter. For it is easier for an
animal to survive when the vital processes are more or less in abeyance.