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At first sight it may seem to the reader that these studies we have just been making in matters concerning the shape of the orbit and the attendant circumstances which regulate the seasons were of no very great consequence; but, in the opinion of some students of climate, we are to look to these processes for an explanation of certain climatal changes on the earth, including the Glacial periods, accidents which have had the utmost importance in the history of man, as well as of all the other life of the planet.
It is, therefore, a fair presumption that large lakes have covered much of its present deserts, and that they have dried up by the operation of the same changed climatal conditions as those which have reduced the Caspian and the Dead Sea to their present dimensions.
We have already noted the fact that the sheep is especially adapted to serve man in high latitudes, where he has to provide against the winter's cold. The camel is an even more striking instance in which the value of the creature depends upon climatal peculiarities. It is peculiarly fitted, by its ancestral training and development, for the use of men who dwell in arid countries.
There are certain peculiarities of Mount Ætna which are due in part to its great size and in part to the climatal conditions of the region in which it lies. The upper part of the mountain in winter is deeply snow-clad; the frozen water often, indeed, forms great drifts in the gorges near the summit.
So far from a gradual progress towards perfection forming any necessary part of the Darwinian creed, it appears to us that it is perfectly consistent with indefinite persistence in one state, or with a gradual retrogression. Suppose, for example, a return of the glacial epoch and a spread of polar climatal conditions over the whole globe.
So far as the observer can judge, the climatal conditions and the other influences which affect the wear of rocks have not greatly varied in the past from what they are at the present day. Now and then there have been periods of excessive erosion; again, ages in which large fields were in the conditions of exceeding drought.
In the case of those species, which have undergone during whole geological periods but little modification, there is not much difficulty in believing that they may have migrated from the same region; for during the vast geographical and climatal changes which will have supervened since ancient times, almost any amount of migration is possible.
A large part of the difficulties which geologists encounter in endeavouring to account for the changes of the past arise from the evidences of great climatal revolutions which the earth has undergone.
But the Miocene and Eocene periods were certainly warm, and these alpine plants could hardly have migrated over tropical forest lands, while it is very improbable that if they had been isolated at so remote a period, exposed to such distinct climatal and organic environments as in Madagascar and Abyssinia, they would have in both places retained their specific characters unchanged.
In considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions. Of late, almost every author who has studied the subject has come to this conclusion.
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