yield of milk in domestic cattle has been increased to a remarkable degree; in the same manner there has been brought about a marked improvement in the yield of wool in sheep, the egg

FIG. 220-Breeds of domestic fowls, showing the results of crossbreeding and selection: A, white-faced black Spanish; B, black-tailed Japanese bantam; C, long-tailed Japanese game fowl; D, Houdins; E, red Pyle games; F, buff Cochins. (After Howard.)

laying capacity of the domestic fowl, and the quality and yield of grains and fruits. The work of the selective breeder has been greatly facilitated by crossing related varieties in order to secure the variations among which the desired selections can be made. It is in this way that Mr. Burbank has accomplished most of

his remarkable achievements in the breeding of plants. In this way there has arisen the manifold variety of dogs which exhibit such diverse types as the terrier, the poodle, the King Charles spaniel, the greyhound, the bulldog, and the St. Bernard. Dogs arose from several wild ancestral varieties or species, and they have been crossbred and selected, consciously or unconsciously, for very diverse qualities. Our horses, cattle, sheep, pigs, cats, and poultry have probably had a multiple origin, and the same is true of many of our varieties of fruits and grains. According to Darwin, our domestic varieties of pigeons, however, are descended from the original blue rock pigeon, Columba livia. Through variation and selection there has now arisen an astonishingly large number of varieties, which differ so markedly in structure and appearance that they would unhesitatingly be ranked as distinct species if found in a state of nature.

Man selects the peculiarities of an organism which he wishes to enhance, but Nature, according to the theory of natural selection, is continually preserving those variations which are of service to the organism in the struggle for existence. Natural selection is always working to perfect the adjustment of the organism to its environment. And if man can effect a remarkable amount of modification in the course of a few generations of selective breeding, how great may be the modifications produced by Nature in the course of the ages!

The term "struggle for existence" was used by Darwin "in a large and metaphorical sense." An organism struggles with others of its species when there is a scarcity of food or insufficient space. It may be said to struggle against enemies and parasites, or any competitors which partake of its means of subsistence. It may struggle against adverse conditions of its environment, such as extremes of heat, cold, or drought. The life of every organism is a battle against various forces of the environment that threaten its maintenance. It is a battle in which the losses are high. Many are called, but few are chosen. But the outcome is a stronger and better adapted race of organisms.

The theory of natural selection has come in for a good deal of

rather sentimental criticism because it explains evolution as the outcome of a gruesome process of selective survival. But if the objectors had only paused to reflect that all organisms which have lived have also died, and that just as many animals have died of starvation and just as many lambs have been eaten by wolves whether the fittest survived or not, they would have perceived the futility of their objection. That we live in a world in which organisms are produced with a prodigal hand and are destroyed by the wholesale is simply a fact of observation. Nature is the theater of strife and destruction whether species are the product of a miraculous creation or the survival of the fittest. The selectionists contend that out of the struggle has come the production of higher forms of life, whereas, according to the opposed standpoint, all the cruelty of Nature, which is so much deplored, results merely in a fruitless waste of life.

The theory of natural selection is primarily a theory of how organisms have come to be modified along adaptive lines. The variations with which it works may be in all directions, and from the standpoint of the interest of the individual they may be good, bad, or indifferent. The bad variations are eliminated; the indifferent ones may be tolerated; and the "good" ones tend to be accumulated. Should it greatly profit an insect, for instance, to be colored like its environment, we may suppose that those color variations which most nearly resemble the surrounding colors would be preserved. Through the accumulations of such favorable variations it may be understood how such cases of protective resemblance as the leaf butterfly might finally have been evolved. To quote Mr. Darwin:

It may metaphorically be said that natural selection is daily and hourly scrutinizing throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working whenever and wherever opportunity offers at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages, and then, so imperfect is our view into long past geological

ages, that we see only that the forms of life are now different from what they formerly were.

Mr. Darwin believed, as is indicated by the passage just quoted, that the formation of species goes on very slowly through the accumulation of small variations. He was familiar with the fact that extensive variations sometimes occur and that they formed the starting point of a number of domestic varieties of animals and plants, but he believed that such extreme variations, on account of their radical departure from the type, would generally be ill adapted and would tend to die out in a state of nature.

One of the important requirements of a theory of the cause of evolution is that it should explain why organisms tend to become more different from each other in the course of their descent. Theoretically, evolution might occur by all members of a group being modified in a given direction, or it might result in different members being modified along diverging lines. We must of necessity suppose that evolution has pursued the latter course in order to account for the great diversity of species found in nature, and the peculiar grouping into which they fall. The tendency to divergence is brought about, according to Darwin, by the fact that competition is most severe between forms which are most nearly alike. Such forms tend to occupy the same kind of habitats and to subsist upon the same kinds of food. Variations which enable an organism to occupy a somewhat different situation would secure a certain amount of relief from the severity of the struggle for existence. Consequently, Nature puts a premium on diversity. This leads to what Professor Bailey has termed "the survival of the unlike." In the struggle for existence it pays to be different. With a diversity of products Nature can support a greater quantity of life. What we might call the pressure of life forces creatures to adapt themselves to all the situations in which a living being can eke out an existence. And when we consider how Nature has peopled the depths of the sea, the sands of the desert, the caves of the

earth, and the frigid regions of the Arctic Circle, and when we reflect upon the curious shifts for a living which organisms have adopted in order to solve the problem of perpetuating their kind, we cannot fail to be impressed with the severity of the pressure which has forced the organic world into all these diverse habitats and modes of living. It is economic pressure which compels people to follow the diversity of employments in which we find them engaged. Men are not sewer diggers or garbage collectors from choice. Most people get their living as they can and not as they will. Consciously or unconsciously, they are forced into a multitude of activities as a result of competition for a means of subsistence. According to Darwin the formation of species is likewise the consequence of the struggle for existence. Evolution tends not only to go upward or, at times, downward, but it has a strong tendency to go sidewise. Hence the branching and the rebranching of the tree of life.

Darwin's theory of how species came to be formed through natural selection is not without difficulties. One of the most serious of these was pointed out by Mr. Fleeming Jenkin in a very able review of The Origin of Species. An incipient variety, according to Jenkin, would not tend to diverge more and more in successive generations; it would be apt to breed with the parental type, and the offspring would tend to do the same. Any differential characters it might possess would sooner or later melt away into the general aggregate, and the new species, therefore, could not get started.

Most of the attempts to meet the objection raised by Jenkin bring into play some form of isolation as a preventive of the swamping effects of intercrossing. Moritz Wagner was one of the first to dwell upon the importance of this factor, and he went so far as to declare that without isolation no speciesforming could take place ("Ohne Isolirung keine Arten"). A study of the distribution of closely allied species shows that there is much truth in Wagner's doctrine, especially if we include under isolation other means of preventing intercrossing besides isolation in space. Undoubtedly barriers have a great deal to