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

BOOK 3

this group were in Baikal from its beginnings. Finding there what was denied them in all other parts of the world, a large and friendly home where there was no competition from more developed crustacean types, such as crayfish and crabs, and shrimps and prawns, they have done their best to take the place of higher Crustacea. In this single sheet of water they have evolved into over three hundred different species—more than as many as are to be found in all the rest of the world—and many of them belonging to purely Baikalian genera. They have launched out into the most varied occupations. There are deep-water Gammarids, blind but with long feelers to compensate for the loss of sight, living at three hundred fathoms ; there are Gammarids which swim all their life in the open water deep below the surface, and are transparent as any jellyfish; there are large shore-living Gammarids, four inches long—sand-hoppers doing their best to be lobsters; and so on. Here again, freedom from competition has allowed the surprising evolution of a group which elsewhere has had to keep its potentialities locked up, unrealized.

Isolation of a piece of land or a body of water from the rest of the world always permits its animal and plant inhabitants to evolve along their own peculiar lines. This is not only true for large groups, like the sandhoppers in Baikal or the marsupials in Australia, but also for genera and species and varieties. We shall have more to say on this subject when we come to discuss the machinery of Evolution. Here we will content ourselves with but two examples, one of which, however, is of considerable historical interest. Those who are interested in the subject can pursue it in Wallace’s famous book Island Life.

If a find of fossil animals on a large continent first put Evolution as a seed of thought into the fertile soil of Darwin’s mind, the germination of that seed was brought about by a problem of present-day distribution on an isolated archipelago. The Beagle visited the Galapagos Archipelago, a collection of some fifteen volcanic islands, separated from each other by distances ranging from a mile or so up to nearly one hundred miles, lying on the equator in the Pacific. he nearest mainland is the west coast of South America, six hundred miles away. The account Darwin gave of them has been supplemented, though not supplanted, by Dr. Beebe’s beautifully illustrated book, Galapagos. hese islands, as Darwin pointed out, resemble the Cape Verde Archipelago,

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THE SCIENCE OF LIFE

CHAPTER 4

off the African coast, in soil, climate, height and size of the islands. In a world deliberately planned and created they would be populated by the same kinds of creature ; what suits one suits the other. But they are not. -Their animal and plant inhabitants are totally different. The inhabitants of the Cape Verde Islands are related to those of Africa, those of the Galapagos to those of America. On the Galapagos “there are twenty-six land-birds ; of these twenty-one, or perhaps twenty-three, are ranked as district species, and would commonly be assumed [when Darwin published this passage, in 1859 !] to have been here created ; yet the close affinity of most of these species to American species is manifested in every character, in their habits, gestures, and tones of voice ”—and so with the other animals and plants of both archipelagos—they are closely related to, though often slightly different from, those of the nearest mainland.

Thus, here we have the same fact, of difference in species but resemblance in general type, which emerged from the fossils of the other side of the South American continent, only now the differences and resemblances concern two sets of creatures separated in space instead of in time.

Such facts at once receive an explanation in terms of Evolution. Chance immigration of storm-pressed birds, wind-blown seeds, tortoises or their eggs drifted in logs or brushwood, would people the archipelago from the continent; this, followed by new evolution in the new and isolated home, would account both for the resemblances and the differences between the inhabitants of the archipelago and those of the neighbouring mainland. But in terms of the Creationist view, there is no explanation.

As our second example we choose St. Helena. St. Helena is perhaps the most isolated spot on the globe, the most insular of all islands. Well over half of its two hundred species of insects are to be found in no other region, and three-quarters of its thirty snails, and four-fifths of its flowering plants : and it boasts no mammals, no land-birds, no reptiles, no amphibia, no fresh-water fish and no fresh-water plants. In other words, animals and plants have either wholly failed to reach this water-girt speck of land; or, if they have succeeded, have usually evolved and changed into something new.

But it is not only the living things which are present in a given region which have testimony to offer us. The absences may be as significant as the presences, just as silence may sometimes be as eloquent as