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female never, perhaps, sees the male which fertilizes her dropped eggs? In many fishes the spring ornamentation of the males is just as marked and just as brilliant as in the birds or other animals of much higher intelligence and corresponding power of choice. Witness the horned dace, chubs, and stone rollers in any brook in spring. Choice on a basis of ornament and attractiveness implies a high degree of aesthetic development on the part of the females of animals of whose development in this line we have no other proof. Indeed, this choice demands asthetic recognition among animals to which we distinctly deny such a development, as the butterflies and other insects in which secondary sexual characters of color, etc., are abundant and conspicuous. Similarly with practically all invertebrate animals. Further, in those groups of higher animals where asthetic choice may be presumed possible, we have repeated evidence that preferences vary with individuals. Certainly they do with men, the animal species in which such preferences certainly and most conspicuously exist. In some human races hair on the face is thought beautiful; in others, ugly. Besides even if we may attribute fairly a certain amount of aesthetic feeling to such animals as mammals and birds, is this feeling so keen as to lead the female to have preference among only slightly differing patterns or songs? Yet this assumption is necessary if the development of ornament and other attracting and exciting organs is to be explained by the selection and gradual accumulation through generations of slight fortuitously appearing fluctuating variations in the males. There are actually very few recorded cases in which the observer believes that he has noted an actual choice by a female. I)arwin records eight cases among birds. Since 1)arwin, not more than half a dozen other cases, all doubtful, have been noted. Also a few instances, all more illustrative of sexual excitation of females resulting from the perception of odor or actions, than any degree of choice on their part, have been listed. In numerous cases the so-called attractive characters of the males, described usually from preserved (museum) specimens, have been found, in actual life, to be of such a character that they cannot be noted by the female. For example, the brilliant colors and curious horns of the males of the dung beetles are, in life, always so obscured by dirt and filth that there can be no question of display to the female eye about them. The dancing swarms of many kinds of insects are found to be composed of males alone with no females near enough to see; it is no case of an excitatory flitting and whirling of many males before the eyes of the impressionable females. Of many male katydids singing in the shrubbery will not for any female that particular song be loudest and most convincing that proceeds from the nearest male, not the most expert or the strongest stridulator? Similarly with the flitting male fireflies; will not the strongest gleam be, for any female, that from the male which happens to fly nearest her, and not from the distant male with ever so much better, stronger light? Even in the human species, propinquity is recognized as the strongest factor in the choice of mates. Several other serious objections can also be urged against the sexual selection theory, but the most important one of them all is that all the evidence (though it is little in quantity as yet, although of good quality) based on actual experiment, is strongly opposed to the validity of the assumption that the females make a choice among the males based on the presence in the males of ornament or attractive colors, pattern, or special structures. Such experiments have been undertaken by Dürigen and Douglas with lizards, and by Mayer with moths. It must be said, however, in closing this brief discussion of the sexual selection theory, that no replacing or substitute theory of anything like the same plausibility has yet been offered to take its place. There is no question that, in many cases, brilliancy of breeding colors, development of processes, and the like, is often correlated with superior vigor. This is especially true among fishes and birds. This reason could, however, not at all account for such structures as the highly specialized stridulating organs of certain insects. The problem of the secondary sexual characters, especially of those which seem to stand in opposition to the natural selection theory, is one of the most pressing in present-day biology.

CHAPTER VI

ARTIFICIAL SELECTION

We can command Nature only by obeying her laws. This principle is true even in regard to the astonishing changes which are superinduced in the qualities of certain animals and plants in domestication and in gardens.-LYELL.

VARIETIES are the product of fixed laws, never of chance. With a knowledge of these laws we can improve the products of nature, by employing nature's forces in ameliorating old or producing new species and varieties better adapted to our necessities and tastes. Breeding to a fixed line will produce fixed results. There is no evidence of any limit in the production of variation through artificial selection; especially if preceded by crossing.-LUTHER BURBANK.

The name Selection has been long used for the process by which breeds or races of domestic animals or plants have been formed in the past, and for the process by which the skillful breeder can develop new forms at will. This latter process, called by Youatt "the magician's wand," by which the breeder can summon up any form of animal which may meet his needs or please his fancy, has been especially designated as Artificial Selection. By it we have derived all of our familiar hosts of varieties of domesticated animals and plants. The similar process in nature was accordingly designated by Darwin, Natural Selection. It refers to the development or increase of traits adaptive or advantageous in the life of a species, through the survival for reproduction of a greater proportion of individuals possessing the characters in question than of those which do not. In any race, it is the individual which succeeds in reaching maturity which determines the future of the race. The qualities of the multitude which die prematurely are naturally not repeated in heredity. In general, the forms pro

duced in artificial selection are not those which could arise or even exist in nature. In nature, hardiness or power of resistance in competition or the struggle for existence is all important. In artificial selection stress is laid chiefly on characters useful or attractive to man. From the standpoint of self dependence, the improvements due to artificial selection constitute a sort of retrogression.

In general, the production of a new race of animals or plants in domestication is the outcome of the work of a number of factors, in which human or artificial selection plays a leading part, a part which increases in importance with the degree of intelligent choice concerned in it.

In the formation of a new race of animals or plants, we may have the following stages or factors:

1. Unconscious selection with more or less complete isolation.

2. Conscious selection of the most desirable individuals. 3. Conscious selec

Fig. 46.-White-crested black Polish cock. tion directed toward

(After photograph.) definite or special ends.

4. Crossing with other races or with other species (known as hybridizing), in order to increase the range of variation, or to add or combine certain specific desirable qualities or to eliminate those undesirable, this accompanied by conscious selection directed toward definite ends. On this series of processes breeding as a fine art must depend.

Taking as an illustration some of the breeds of medium wool sheep found in Southern England: we have (1) the domestication of sheep in each of the different counties or natural

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areas. In the beginning men are satisfied with sheep as sheep. Little attention is paid to the distinction among individuals. Those which are feeble, ill nourished, untamable, scant-fleeced, or otherwise unfit will be eliminated, a process which will tend to improve the stock, without giving the race distinctive qualities, except as compared with the wild original. To form distinct races, the factor of isolation must enter. Those in one county, for example, will be, at the beginning, somewhat different from those in another. Each herd will show its own traits in time, these due primarily to differences in the original stock, secondarily to the predominance of one form of variation over others. Exchanges of sheep will, by cross-breeding, tend to unify the type of sheep in some one county, or on some side of a barrier across which sheep are not driven. With this, there will be also variations in the character of the unconscious selection. One type of sheep will flourish in a meadow county, another on a moor, and still another on the Fig. 47.- Silver-laced Wyandotte cockerel. rocky hills. At any rate, (After photograph.) as the environment varies, so will the character of the selection. Thus as a final result, in Southern England, the Southdown sheep of Sussex have tawny faces and legs: the sheep of Hampshire have black faces, ears, and legs, with a black spot under the tail; this black spot is lacking in the sheep of loevon. In the Cheviot sheep the face and ears are white, the head free from wool, while the ears, unlike those of most of the others, stand erect. In the dun-faced Shropshire sheep, the faces are more or less covered by wool. All these are hornless, while the more primitive Dorset sheep with white face and ears

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