ratio—that is, by addition; therefore, whatever may be the ratio of increase, a geometrical progression will sooner or later outrun an arithmetical one. Hence sooner or later the world must be overstocked, did not vice, misery, or prudence come in as checks, reducing the ratio of multiplication. This law has been criticised as a partial truth, so far as man is concerned. This means simply that there are factors also in evolution other than those recognized by Malthus. Nevertheless, Malthus's law is a sound statement of one great factor. And this law is simply the expression of the struggle for existence as it appears among men.

The doctrine of organic evolution was first placed on a firm basis by Darwin, because Darwin was the first who clearly defined the force of natural selection. Darwin, however, recognized other factors, known or hypothetical, and was interested more in showing the fact of descent and one cause of modification than in insisting on the all-sufficiency of the cause especially defined by himself.

In later times, Weismann and his followers have laid more exclusive stress on natural selection and its Allmacht or exclusive power in bringing about organic evolution. This view is known as Neo-Darwinism and the school of workers who profess it as Neo-Darwinians. Few investigators question the far-reaching influence of natural selection, but there are many phases in organic evolution which cannot be ascribed to it. Hence the search for other factors has been assiduously prosecuted, and doubts of Darwinism have been widely expressed; but this doubting has been thrown not so much on the Darwinism of Darwin, nor, as a rule, on the law of natural selection, but rather on the Allmacht claimed for it by Weismann and his associates.

Without attempting any elaborate discussion of questions still far from settled we may venture these suggestions:

1. Given the facts of individual variation, of inheritance, and some check to freedom of migration, natural selection would accomplish some form of organic evolution; species would be formed by the survival of the adapted, adaptations would be perpetuated, and minor differences would develop in time into deep-seated differences.

2. With natural selection alone, however, the actual facts in organic evolution as we know them would apparently not be achieved.

3. In other words, while natural selection furnishes the motive force of change, other influences, extrinsic and intrinsic, help to direct the channels in which life runs. It is necessary to consider other causes for the great body of indifferent characters or traits not produced by adaptation, and apparently not yielding either advantage or disadvantage in the struggle for life.

4. The formation of species of animals and plants through natural selection finds an analogy in the formation of rivers through gravitation. Gravitation is the motive power carrying the waters from the uplands to the sea. The courses of streams are determined by a number of minor influences acting in concurrence with gravitation, the final result far more complex than the single cause would produce.

5. In like fashion, while natural selection is the motive element in descent or evolution, the total result is due to a concurrence of causes, and is too complex to be explained by natural selection, by the principle of utility, or the survival of the fittest alone, and the varying effects must be ascribed to a variety of causes.

Certain minor traits, as color patterns, relative proportions of parts, survive-apparently without special utility, but because these traits were borne by some ancestors or group of ancestors. This has been called the Survival of the Existing. In making up the fauna or flora of any region those organisms actually present when the region is first stocked must leave their qualities as an inheritance. If they cannot maintain themselves their breed disappears. If they maintain themselves in isolation their characters remain as those of a new species. In hosts of cases, the survival of characters rests not on any special usefulness or fitness, but on the fact that individuals possessing these characters have inhabited or invaded a certain area. The principle of utility explains survivals among competing structures. It rarely accounts for qualities associated with geographic distribution. The nature of the animals which first colonize a district must determine what the future fauna shall be. From their actual specific characters, largely traits neither useful nor harmful, will be derived for the most part the specific characters of their successors.

It is not essential to the meadow lark that he should have a black blotch on the breast or the outer tail feathers white. Yet all meadow larks have these marks, as all shore larks possess

the tiny plume behind the ear. Any character of the parent stock, which may prove harmful under new relations, will be eliminated by natural selection. Those especially helpful will be intensified and modified. But the great body of characters, the marks by which we know the species, will be neither helpful nor hurtful. These will be meaningless streaks and spots, variations in size of parts, peculiar relations of scales or hair or feathers, little matters which can neither help nor hurt, but which have all the persistence heredity can give.

In regard to natural selection our knowledge seems positive. In regard to most other factors of organic evolution we have to deal so far not with clearly demonstrated facts but with "probabilities of a higher or lower order," their value to be ultimately shown by experiment.

In this connection the following words of Dr. Edwin Grant Conklin are very pertinent:

"On the whole, then, I believe the facts which are at present at our disposal justify a return to the position of Darwin. Neither Weismannism nor Lamarckism alone can explain the causes of evolution. But Darwinism can explain those causes. Darwin endeavored to show that variations, perhaps even adaptations, were the result of extrinsic factors acting upon the organism, and that these variations or adaptations were increased and improved by natural selection. This is, I believe, the only ground which is at present tenable, and it is but another testimony to the greatness of that man of men that, after exploring for a score of years all the ins and outs of pure selection and pure adaptation, men are now coming back to the position outlined and unswervingly maintained by him."

Finally we ought not to suppose that we have already reached a satisfactory solution of the evolution problem, or are, indeed, near such a solution.

"We must not conceal from ourselves the fact," says Roux, "that the causal investigation of organisms is one of the most difficult, if not the most difficult, problems which the human intellect has attempted to solve, and that this investigation, like every causal science, can never reach completeness, since every new cause ascertained only gives rise to fresh questions concerning the cause of this cause."

In order to explain certain important phenomena outside the apparent range of natural selection, a theory of another sort of selective activity is recognized by many biologists. This is the theory of Sexual Selection first propounded by Darwin.

Differences between male and female individuals of the same species are the rule rather than the exception (Fig. 40). Many of these differences are what might be called the necessary ones due to the particular functions assumed by each individual in this differentiation of sex. Of this nature are, besides those fundamental ones of the primary reproductive ones, such others as

those specially connected with the care and rearing of the young; as the mammæ of female mammals, the brood pouches of the female kangaroos and opossums, etc. But a moment's reflection calls to mind the existence of a host of other differences between males and females of the same species which plainly have no such immediate relation to the distinct functions or duties assumed by each in the business of production and care of young. For example, the long plume feathers of the male bird of paradise, the curious chitinous horns of the male leaf-chafer beetles (Fig. 41), the brilliant plumage of many male birds as contrasted with the sober dress of the females, and a host of other distinguishing characteristics of the sexes in many animal species. Now these differences are all conveniently named by the phrase "secondary sexual differences," and the explanation of their origin has come to be one of the most

FIG. 41.-Male and female Scarabeid beetles, Phaneus mexicanus, showing sex dimorphism; the male with prominent dorsal horn on head. (From specimens.)

puzzling of biological problems. The most familiar and, for many years, a widely accepted solution of this problem, is that embraced in the theory of sexual selection proposed and fought for by Darwin and Wallace, but later discarded by the latter of these great naturalists.

Before taking up the sexual selection explanation of distinguishing sex characters, it is well to pay a little further attention to the characters themselves. And for this purpose a rough grouping or classification may be attempted.

The characters may be of special use to the possessor (male or female) or for the benefit of the young, such as weapons of offense and defense (antlers of male deer, stings of female bee and wasp, tusks of male swine, etc.), or special organs for mating (seizing and holding organs of certain male crabs, suckerlike holding pads on the feet of male water beetles (Fig. 42), or special locomotory organs (presence of wings in the male and their