t 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.

Disappearance of many Ancient Types

We cannot leave the last period of the Palaeozoic era and its prolonged
ice age without noticing that it meant the entire cessation of a large
number of ancient types, especially among plants and backboneless
animals, which now disappear for ever. It is nece