Darwinian Theory
the explanation of the working of natural selection in effecting specific changes in plants and animals. "Darwinism" must not be confused with "Evolution." Darwinism is restricted to one particular interpretation of the mechanism of the universe, and is essentially stated in Darwin's great work, "The Origin of Species by Means of Natural Selection." Din Outline of Origin of Species. To gain insight into the means of modification, Darwin begins with a study of the variation of plants and animals under domestication. Those who admit the unity of domestic races should be cautious in denying the unity of the wild ones. mestic races all exhibit adaptations to man's use or fancy, rather than to their own good. The key to this is man's power of selection. Nature gives successive variations, man accumulates these, so making for himself useful breeds, and often (e. g., in sheep, cattle, roses, dahlias) profoundly modifies their character even in a single human lifetime; so that in all characters to which he attends, they may differ more than the distinct species of the same genera. Unconscious selection, which results from everyone trying to possess and breed the best animals, is even more important than conscious selection. Two flocks of Leicester sheep kept equally pure appear of quite different varieties after 50 years. Such slowly accumulated change explains why we know so little of the origin of domestic races; and its absence in regions inhabited by uncivilized man explains why these yield no plants worth immediate culture. Human selection is facilitated (1) by the keeping of large DARWINIAN THEORY numbers, since variations will be more frequent; and (2) by preventing free intercrossing. Some species vary more than others.
Variation Under Nature. - No two blades of grass are alike, and far more marked differences often occur, several strains or varieties sometimes existing in the same species. Between these strains , and much more frequently between forms which systematic botanists and zoölogists rank as true species, perfectly intermediate forms may occur. No agreement about the definition of species (the amount of difference necessary to give any two forms specific rank) has ever been reached. Individual differences are of the highest importance, as the first steps toward the slightest varieties worth recording; these in turn toward more distinct and permanent varieties; these varieties again toward sub-species, and in the next stage to species, though extinction may often arrest the process. The species which present most varieties are those which have the greatest geographical range, or the widest diffusion in their own territory, or which possess the greatest number of individuals.
Struggle for Existence. All organic beings tend to increase with extreme rapidity, so that if they were not kept down, the earth would soon be covered by the progeny of a single pair. Since organisms are reproducing themselves so rapidly, and not all their offspring can escape their enemies, get food and live, much less leave progeny in turn, there must in every case be a struggle for existence, either of one individual with another of the same species, with the indiother of the same species, species or with the physical conditions of life; often with all these at once, and that more or less intensely throughout the whole duration of life. The checks which prevent increase are more obscure, and vary in each case.
In all cases the amount of food gives the limit. The youngest organisms generally suffer most. The struggle for life is most severe among individuals and varieties of the same species, and among the species of the same genus, since these tend to fill the same place in the economy of nature. The structure of every being is related to that of the others with which it competes, or from which it seeks to escape, or on which it preys.
Natural Selection. _ The preservation of favorable variations, and the destruction of injurious ones, is termed by Darwin "Natural Selection," or less figuratively by Spencer, the "Survival of the Fittest." Human selection acts only for man's own good, on mere external and visible characters, and irregularly throughout a short period; natural selec- 18-Vol. III-Cyc DARWINIAN THEORY tion acts for the good of the being itself, on the whole machinery of its whole life, and incessantly on the species, throughout almost infinite time. The circumstances favorable to the production of new forms are great variability; large numbers of individuals; the complex effects of intercrossing; isolation in small areas; also extension over continental ones, especially if these vary in altitude; and considerable lapse of time. Rare species are shown to be in process of extinction. The divergence of character in domestic breeds, largely due to the fact that "fanciers do not, and will not, admire a medium standard, but like extremes," applies throughout nature from the circumstance that the more diversified the descendants from any one species become in structure, constitution and habits, by so much will they be better enabled to seize on many and widely diversified places in nature, and so to increase in numbers. A carnivorous animal which has reached the maximum numbers its territory can support, can succeed in increasing only by its varying descendants seizing places hitherto occupied by other animals. This must hold equally of all species, and is separately demonstrated for plants.
Sexual Selection. - Not merely do individuals struggle for existence, but the males struggle for the females, and the most vigorous tend to leave most progeny. Several weapons, offensive and defensive, like the cock's spurs, the stag's horns, or the lion's mane, are used in this struggle, and the most useful variations are those which are transmitted.
DASKAM but in 1862 the long-tailed and intensely reptilian bird Archæopteryx was discovered, while in 1875 the researches of Marsh brought to light certain cretaceous birds, and the hypothesis of Darwin is thus admirably verified.
Geographical Distribution. - Neither the similarity nor the dissimilarity of the inhabitants of various regions, whether of land or of sea, can be accounted for by identity or differences of climate, or other physical conditions; but both are related in the most striking degree to the absence or presence of barriers to migration between those regions. Morphological Arguments. The physiological and distributional lines of argument furnished by morphology are mainly four, and are derived from (a) Classification, (b) Homologies, (c) Embryology, (d) Rudimentary Organs. The great fact of classification is that organic beings, throughout all time, are arranged in groups subordinated under other groups, individuals under varieties, and these again under species; species under genera; genera under sub-families, families, and orders; and all under a few grand classes. The element of descent is already used in linking all the sexes, ages, forms, and varieties of the same species, widely though these may differ from each other in structure.
The members of the same class, independently of their habits of life, resemble one another in their general plan of organization. Thus the hand of man, the digging-paw of the mole, the leg of the horse, the paddle of the porpoise, and the wing of the bat, are all constructed on the same pattern, bone corresponding to bone.
Laws of Variation. The same laws appear to have acted in producing the Serial Homology is that unity of type lesser differences between varieties of the which is found on comparing the differsame species and the greater differences ent parts and organs in the same individbetween species of the same genus. Spe- ual. The complex and varied jaws and cific characters are more variable than legs of a lobster, or the different leaves, mentary organs and secondary sexual sepals, petals , stamens, and pistils of a Zebra-like flower, are all found to be modifications of a simple limb, and a simple leaf-organ development cific characters are more variable than characters are variable. stripes on horses, or wood-pigeon's markings on fantails, tumblers, etc., may be explained as reversions toward their ancient progenitors. etc., may be respectively. The process of d goes from the general to the special; thus there is generally an advance in organization. In peculiar conditions degeneration may occur. All these facts are explained on the principle of successive slight variations not necessarily or generally supervening very early in life, and inherited at corresponding period; hence it is in the highest degree probable that most embryonic stages show us more or less completely the progenitor of the groups in its adult state; and embryology thus rises greatly in interest.
JOSEPHINE a Geological Succession of Organic Beings. The most ancient forms differ widely from those now living, yet frequently present characters intermediate between groups now widely divergent, and resemble the embryos of the more recent and more highly specialized animals belonging to the same classes. Darwin's belief that the distinctness of birds from all other vertebrates was to be accounted for by the extinction of a long line of progenitors connecting them with reptiles, was in 1859 a mere assumption; (MRS. SELDEN BACON) an American DASKAM, DODGE DASYPROCTA novelist, born in Stamford, Conn., in 1876, and graduated from Smith College in 1898. She first attained prominence by her novel "The Madness of Philip," published in 1902, in which she commented upon some aspects of primary education. Her works include: "The Biography of a Boy" (1910); "The Inheritance" (1912); "The Luck o' Lady Joan" (1913); "Today's Daughter" (1914), etc.