Page:Popular Science Monthly Volume 28.djvu/81

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is in giving a preference to "listed" securities. Many persons seem to think stocks and bonds must have a value if they are quoted at some stock exchange. On the contrary, such a position is likely to expose them to manipulation for purely speculative purposes. Stock-exchange quotations, as a rule, are unsafe guides to buyers. Every security must stand on its own merits, and purchasers have merely to follow business principles as taught by the canons of common sense.


EVERY group of organisms, every genus and every species of plant or animal, has certain strong points which enable it to hold its own in the struggle for existence against its competitors of every kind. Most groups have also their weak points, which lay them open to attack or extinction at the hands of their various enemies. And these weak points are exactly the ones which give rise most of all to further modifications. A species may be regarded in its normal state as an equilibrium between structure and environing conditions. But the equilibrium is never quite complete; and the points of incompleteness are just those where natural selection has a fail* chance of establishing still higher equilibrations. These are somewhat abstract statements in their naked form: let us see how far definiteness and concreteness can be given to them by applying them in detail to the case of a familiar group of agricultural plants—the clovers.

To most people clover is the name of a single thing, or, at most, of two things, purple clover and Dutch clover; but to the botanist it is the name of a vast group of little flowering plants, all closely resembling one another in their main essentials, yet all differing infinitely from one another in two or three strongly marked peculiarities of minor importance, which nevertheless give them great distinctness of habit and appearance. In England alone we have no less than twenty-one recognized species of clover, of which at least seventeen are really distinguished among themselves by true and unmistakable differences, though the other four appear to me to be mere botanist's species, of no genuine structural value. If we were to take in the whole world, instead of England alone, the number of clovers must be increased to several hundreds. The question for our present consideration, then, is twofold: first, what gives the clovers, as a class, their great success in the struggle for existence, as evidenced by their numerous species and individuals; and, secondly, what has caused them to break up into so large a number of closely allied but divergent groups, each possessing