The evolution of teeth in mammalia presents a problem which calls for an analogous feat of inventive genius. As the problem is representative of many others it is worth consideration. The study of these teeth is a specialty of Professor Henry F. Osborn's, and though to the layman this may seem a very small matter it is really big enough to concern not only science but philosophy.
Anyone who will look into the glass at his back teeth, the molars or grinders, will perceive that their tops are not flat but raised into little promontories, tubercles, or "cusps." An eye-tooth, on the other hand, is a single sharp peg or fang.
Were the molars, then, far back in evolution, made by fusing together two or three original peg-shaped teeth, each component being now represented by a cusp? Or were they always single, each growing its own several cusps for grinding purposes?
Professor Osborn has shown that the latter was the case.
We used the words "for grinding purposes." That was raising the window. It has been raised before. Once in a long while a biologist gets out. As a rule however they will not even see it, or, seeing it, they deny that it is a window. If these words, implying something possessed of the purposes, conscious and capable, will not do, how came the cusps to grow? How came the original sharp peg tooth, a cutter and piercer, to broaden and tuberculate its top so as to form, with its opposing fellow in the other jaw, a pair of convenient grinders?
According to the Darwinian theory all sorts of small chance variations, useful and useless, are constantly appearing among the progeny of all species. The useful ones, conferring an advantage in the struggle for existence, persist. The others do not. The usefulness is the cause of the persistence. In scarce seasons an animal that had, for example, developed opposing grinders among its teeth would be able to utilize food not available for the mere cutters. It would tend to live—and therefore produce offspring—while they died. The grinders being handed on by heredity, their usefulness would in time secure the whole field for their owners. A new and predominant species would have arisen, to live until ousted by a stronger.
But this would only apply to variations useful from the moment of their appearance. If at first—as they often are—so small as to be useless, a mere tendency or suggestion, they would not persist. Having, according to the theory, no special purposive force behind them, and being the products of mere accident, they would quickly be diluted out of existence.
The chance theory would therefore be able to account for the persistence of such few variations only as were useful from their first appearance. Are there any such variations? According to the theory itself, no! For it does not admit sudden jumps; merely fine shadings from the common type. And these fine shadings confer no advantage. Since, moreover, they occur only by some chance confluence of conditions, they must depend for their force of heredity upon the continuance of this confluence. And to account for the next, and the next, degree in the progression, the theory must require that the conditions become more and more effective—and so on, till the degrees sum up to a useful degree.
What a lot of wriggling to escape the conclusion that there is a purposive force at work! Even Professor Osborn does not see it in his studies of teeth, though he walks straight up to it. Mr. Gruenberg, summarizing the Professor's work in The Scientific American says: