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THE EVIDENCE FOR EVOLUTION

The idea of Evolution is an old one. It is older than the Darwinian hypothesis; it is older than Lamarck, who published his particular theory in 1809, the year that Darwin was born; it is older than Buffon or Kant. In a fairly definite form it is as old as Aristotle. The Evolution idea has thus itself evolved, and is the product of many centuries of thought. Yet it was only the last generation that began to give the idea serious consideration, and it is perhaps only the present that has granted it any general measure of acceptance; and it was Darwin who wrought this change, who raised the conception of Evolution from the status of a vague speculative idea to that of a well-grounded theory, which appeals to the majority of educated minds as satisfactory and reasonable.

We do not here propose to sketch the development of the idea, either before or after Darwin; but only, in the first place, to state the grounds on which the belief in Evolution is based, and, in the second, to trace roughly the lines along which animal Evolution has proceeded. In the first few pages of this book, then, we shall endeavour to bring forward some of the evidence on which the modern Evolution theory rests.

Evolution

	First Appearance of Types.	Dominant Types.
Modern	...	Man	}	Post-tertiary, 1/2 per cent.
Diluvium	Man	...
Pliocene	...	}	Mammals	}	Tertiary or Cænozoic, 2-1/2 per cent.
Miocene	Monkeys
Oligocene	...
Eocene	Lemurs
Cretaceous	Higher mammals	}	Reptiles	}	Secondary or Mesozoic, 11 per cent.
Jurassic	{	Birds Marsupials
Triassic	Monotremes
Permian	Reptiles	Amphibians	}	Primary or Palæozoic, 32 per cent.
Carboniferous	Amphibians	}	Fishes
Devonian	Lung fishes
Silurian	Lower fishes	...
Cambrian	...	...	}	Archäen, 54 per cent.
Laurentian	...	...

Fig. 2.—Table showing the chronological succession of the stratified rocks, the subdivision of geological time, the approximate position of the earliest fossils of each of the main types of vertebrates, and the period of domination of each group.

As our first witness, we may call the rocks which constitute the outer portion of the earth, and ask them to tell us what they remember of the history of life upon the planet. We cannot hope

for the whole truth from them, for their memory is imperfect; and yet they can tell us a great number of important facts.

THE EVOLUTION OF THE HORSE

Fig. 3.

From The Guide to the American Museum of Natural History.

From the time when the world was sufficiently cooled for water to condense on its surface, a continual process of unbuilding and rebuilding of rocks has gone on. Wind and water, heat and cold have laid their hands to the work, making sand and dust and gravel out of solid stone, and these products of their labours have been carried off to other places, laid down, and cemented together into new rocks. We do not know the exact age of any particular rock that has been made in this way, nor how long the process has been going on. At a rough guess it may be three or four hundreds of millions of years. The chronological succession of the different rock formations is, however, known, and their relative ages may be judged with considerable accuracy. Here and there, as time went on, the body of a plant or an animal was deposited in the sand or mud or chalk, and has remained in the resulting rocks, in the form of a fossil, through all the ages. If, then, we study the occurrence of fossils in this succession of deposits, we ought to get some indications as to the inhabitants of the globe at various stages of its history. And if we do so, we meet unmistakable evidence that the lower and simpler types, both of animals and of plants, were in existence before the higher. Fig. 2 shows the facts with regard to the vertebrates, the great upper class of the animal kingdom. The first appearance of vertebrate fossils is in the Upper Silurian rocks, that is to say, somewhere after the middle of geological time. The fossils represent the lowest group of fishes. In the next great formation, the Devonian, fossils of two higher groups of fishes are to be found. The first land vertebrates, the amphibians, are doubtfully represented in the upper or newer layers of the same formation, and definitely so in the next, the Carboniferous. Towards the end of the Carboniferous or early in the Permian epoch, the first reptiles appear, and in the following period, or after about three-fourths of geological time had passed, the earliest fossils of mammals occur. The significance of this sequence will become plainer when

the differences and likenesses of these various groups are explained. Each of these great groups in turn formed the dominant animal population of the globe, and each in turn was superseded, although not entirely, by the next. The mammal group itself appears to be on the wane, overcome in the struggle for dominance by its own latest and most remarkable member, man himself.

Fig. 4.

From The Feathered World.