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

THE EVIDENCE FROM LIVING THINGS

so different that they are obviously not members of the same species. And there are animals just different enough to make one wonder whether they should be called different species or not—and this is where the nice problems arise. Authorities vary widely in the freedom with which they erect new species ; some group animals together whenever there is any doubt at all, while others put two specimens in separate species almost whenever they can see a noticeable difference between them. In an extreme instance, the British brambles and roses have been classified as sixty-two species by one authority and as two by another of equal eminence.

For there is no absolute criterion of species, no feature that will stamp living things definitely as the same or different. It used to be thought that there was a distinction between creatures which would breed together and produce fertile offspring and creatures which would not ; the former were members of the same species and the latter were not. But in fact there is every conceivable stage, in some organism or other, between mutual fertility and complete mutual sterility. At one extreme the males and females breed freely together, and their offspring are fertile; in this case they are generally called the same species—mankind will serve as an example. At the other extreme the male and female do not recognize each other as similar creatures ; a lion and a cow will illustrate this. But in between those two extremes there are various links ; there are cases where a male and a female are friendly together but show no desire to breed, cases where they come together but are completely sterile, cases where they produce a few weakly or abnormal offspring, cases where they produce healthy but sterile offspring, cases where their offspring are healthy and fertile but their grandchildren are weak and unhealthy. Where in this series are we to draw the line ? Which of the graded stages are sufficiently fertile to be called members of the same species ?

Indeed, there are plenty of examples of animals and plants which are considered to be different species interbreeding freely in Nature. The case of the carrion. crow and hooded crow is classical. These birds inhabit the greater part of Northern Europe and Asia, but the two species divide this vast area between them. The carrion crow (or common crow) is found in England and South-Western Europe and also in the eastern half of Siberia. The hooded crow is found in North-Eastern Europe and from thence

across to the middle of Siberia and down as far as Egypt. There are therefore two lines along which the species meet: the western boundary runs through Scotland, Denmark and the Elbe Valley to Northern Italy, while the eastern boundary runs down through Siberia. In these boundary zones the two species frequently breed together and produce every possible intermediate stage between the coal-black carrion crow and the hooded crow with its grey body and black head, wings, and tail. A case with specimens of the two species and their intermediates may be seen in the Natural History Museum, London. A similar example is afforded by the flicker, a common North American woodpecker. There are two perfectly distinct kinds of flicker, the common flicker on the eastern side and the western flicker on the west; they differ conspicuously in a number of points in their coloration. Butalong a broad zone running from British Columbia to Galveston the two kinds mix and interbreed freely, and in this boundary region every conceivable kind of intermediate is found. One could multiply similar examples indefinitely ; one more must suffice. In East Africa there are two species of antelopes of the hartebeest kind which, until recently, inhabited different districts and also were always perfectly distinct in such points as the shape of their horns ; but a few years ago they spread towards each other and began to interbreedpresumably some sort of barrier that had been keeping them apart broke down—and now there are all sorts of intermediates between the two. So that the old fertility-criterion of species can no longer be said to work. _

All of this was very perplexing to biologists in the days when they believed living things to have been created in a fixed number of immutable kinds in the garden of Eden. The Creator, they thought, had made so many different species, and it was the business of the systematist to recognize and identify those species and to base his classification upon them. The great Linnzeus, for instance, laid down as biological dogma that “ the number of species is as many as the different forms created in the beginning.” Even in the nineteenth century Cuvier, as Professor J. W. Gregory remarks, “‘ believed that species are as distinct as the different makes of boots sent out from a factory.” _

But it came to be realized that in many cases it was extraordinarily difficult to recognize absolute distinctions of this sort ; to tell, for example, whether there were both Sumatran and Manchurian tigers in Eden or whether there was one tiger which begat

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