Heat, cold, food supply, geographical distribution, winds, water supply, prevalence of certain birds or other animals,-all these are the factors which must be taken into consideration when studying this problem and in trying to trace out definite results.

Huxley, in defining Darwin's hypothesis, says: "As I apprehend it, it is that all the phenomena of organic nature, past and present, result from, or are caused by, the interaction of those properties of organic matter, which we have called Atavism and Variability, with the Conditions of Existence; or, in other words-given the existence of organic matter, its tendency to transmit its properties, and its tendency occasionally to vary; and, lastly, given the conditions of existence; by which organic matter is surrounded-that these put together are the causes of the Present and of the Past conditions of Organic Nature."

Darwin himself says: No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he makes due allowance for our profound ignorance in regard to the mutual relations of all the beings which live around us. Who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare? Yet these relations are of the highest importance, for they determine the present welfare, and, as I believe, the future success and modification of every inhabitant of this world. Still less do we know of the mutual relations of the innumerable inhabitants of the world during the many past geological epochs in its his

tory. Although much remains obscure, I can entertain no doubt, after the most deliberate study and dispassionate judgment of which I am capable, that the view which most naturalists entertain, and which I formerly entertained—namely, that each species has been independently created-is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that natural selection has been the main but not exclusive means of modification."

It will be seen, then, that the various forms of nature, animal or vegetable, are continually changing, affected by the conditions or circumstances which surround them, and that evolution is the term applied to the change.

How Change is Produced-Vast Eras of Time-The Age of the Earth-Evidences of Evolution-Extinct Animals.

N the previous chapter we have seen that Darwin claims that in nature there is a tendency to variability, certain conditions producing it, and that all the diverse forms of animal life are to-day the result of this process. Without an adequate idea of geological time this cannot be realized; but

if we imagine that these changes have gradually been going on for unknown ages, periods which the mind of man cannot grasp, we begin to see the light ahead that Darwin saw.

In the present age geology and palæontology, or the history of extinct animals, through the investigations of Professors Cope, Marsh, Leidy in this country,

England, have become
Though there is still a

and others in Germany and remarkably well understood. vast area unexplored, we know enough of the ancient earth and its history to enable us to view it comprehensively. If we could make a section of the crust we should find the rocks arranged layer by layer. At a certain depth would be what is known as the Eozoic rocks, which if they have contained life do not show evidences of it. Above this we find a layer containing shells and low forms, and so on up, layer after layer, for thirteen thousand feet in New York State, or nearly two miles of solid rock, nearly all the strata being characterised by peculiar forms of life. If we examine these, one after another, up through the crust, we notice that there appears to be an elaboration of structure the higher up we proceed. Thus in the Devonian, or just below it, we first find back-boned animals-fishes; then in a succeeding age reptiles; then milk-giving animals, and finally man. In other words, there has been an apparent evolutionary movement here, so that viewing the results we can easily imagine that one who had studied the rocks as did Darwin might arrive at the conclusion that he was looking at the different steps in the evolution of life.

How long was the thirteen thousand feet in New York forming? The mind fails to grasp the figures, nor can we realise it without taking up instances that are well defined and familiar. In one of the Western States there is an elevation, known as Amethyst Mountain, a mile or more high. One side has been worn away by the elements, so that it appears to

Growth of Forests.

185

have been cut down as with a knife, leaving a more or less sheer precipice. On the face of this can be counted twenty or thirty distinct forest levels. Thus at the base a forest is visible of trees standing upright; the trunks of large size but broken off about ten feet from the ground and completely silicified or turned into stone. The roots, like stone snakes, can be readily traced, while the trees and sections of trunks are so true to life that they might have been buried but yesterday. On top of these trees is seen a stratum of rock, which has given growth to another forest, which attained maturity and died down to form the birthplace of still another. So one after another these forests are piled up to the height of a mile. The reader is familiar with the time entailed in the growth of a forest of to-day, from saplings to trees of the largest size, hence can form some idea of the time which must have elapsed while these forests were dying and growing one upon another.

Those who have visited England and are familiar with the chalk cliffs of Dover have interesting testimony of the duration of time. This vast deposit is simply an ancient ocean bottom almost exactly identical with that being formed in the Atlantic to-day. By some convulsion of nature it has been elevated, and stands as a monument to the vast results attained in nature by the agency of minute organisms through long eras of time.

When the Atlantic cable was taken up for repair, it was found covered with a mud almost entirely made up of minute shells of rhizopods; and later investigations into the ocean bottom showed that