The science of life : fully illustrated in tone and line and including many diagrams

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THE EVIDENCE FROM LIVING THINGS

tions, yield knowledge as definite as any which he obtains, though it refers to epochs a hundred thousand times as remote. The fossils, dated by the rocks, reveal the actual story of life’s past changes. The story does not go back to life’s first beginnings, nor does it cover all kinds of living things ; but for the latter half of life’s existence, and for the most elaborate and interesting of life’s children, it is reasonably complete. And the story the fossils reveal is one of steady evolution, of progressive change, of multiplication and divergence of forms of life, of extinction of one type and its replacement by another. The fossils are the remains of creatures other than those of to-day, which once were alive, living a different kind of life in different surroundings. They testify to the past of animal and plant life in the same direct way as do the mummies of the Pharaohs, or the baked-clay bills and receipts of pre-historic Babylon, or the slaughtered retinue of the King of Ur, to the past existence of human beings who lived very different lives from ours; they testify to the evolution of life as directly as do the discoveries of archeology to the evolution of human culture.

Then there are the indirect evidences. There are the similarities of general plan which are not to be accounted for save by the descent from a common ancestor of all the animals showing the plan. The similarity of plan implies common descent ; the differences in detail imply descent with modification. There are the useless vestiges which yet correspond rigorously with organs that are indispensable in other animals, inexplicable if their possessor be not descended from some ancestor in which the now useless organ had its use. There are the extraordinary phases of the individual’s development in which it passes from one strange likeness to another—likenesses to other creatures, often remote and primitive, to which the adult animal no longer betrays any resemblance, whether in plan or in mode of life. These resemblances are meaningless, and, indeed, deceptive, if they are not recapitulations of ancestral phases in which the race once continued for long periods of past time, though our modern creature hurries through them on its way to its own new and different adult life.

All these facts are inexplicable on any theory of special creation. Save Evolution, no rational explanation of them has ever been put forward ; and on the evolutionary view not only are they explicable, but full of meaning.

We _teviewed the evidence from life’s variability. If we laid more emphasis upon it than has often been done in the past, this is because the evidence from variability, like that from fossils, has of recent years increased enormously, both in account and still-more in cogency. Now that a number of groups like birds and mammals and butterflies have had their minutest varieties classified and the details of their distribution tabulated, the old idea that species are the most real and definite units of life, or even that they are real and definite units at all, sharply marked off from other kinds of units, has gone by the board. There do exist some sharply circumscribed speciesunits; but other such units intergrade or interbreed with one another. ee Tes There is no crucial test by which we can distinguish between a local race or a sub-species and a species, or between a species and a_ genus. There is often disagreement among systematists themselves as to whether a particular kind of animal or plant shall be classified as a full species or a mere variety. There exist interbreeding groups so variable that we would regard the extremes of variation as different species did we not know of the existence of all the intermediates. All this lack of sharp lines and clear limits is to be expected if life’s method is Evolution; but on the Creationist assumption it is chaos and confusion. ok.

The considerable degree of variability to be found in all wild forms of life was emphasized, and the conclusions to be drawn from this fact were driven home by an appeal to the astounding changes which man has been able to bring about in his domestic animals and plants. If greyhound, bulldog, toy terrier, and St. Bernard can all be formed out of wild-dog material in a few thousand years, then that living material is of an extraordinary plasticity, and will lend itself willingly enough to change and evolution.

And finally we have recited some of the facts of the distribution of animals and plants, and have shown that they, too, fall

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Fig. 150. A man at about two months of true: age. Front view of a human

embryo, four-fifths of

an inch long, about

seven months before birth.