Natural The natural process which results in the preservation of favorable variations and the elimination of injurious ones, is termed by Darwin 'Natural or less figuratively by Spencer, the °Survival of the Fittest.* No country can be named where the native inhabitants are perfectly adapted to their conditions and competitors, for as some foreign ers have taken firm possession in every country, we may safely conclude that the natives might have been modified with advantage to resist them. Human selection acts only for man's own good on mere external and visible charac ters, and irregularly throughout a short period; natural selection 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. It leads to the improve ment of each creature in relation to its organic and inorganic conditions of life, and conse quently in most cases to what must be regarded as an advance in organization. The circum stances 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 alti tude or climate, 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 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 thty 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 num bers its territory can support, cannot succeed in increasing unless its varying descendants seize places hitherto occupied by other animals.
This must hold equally true of all species and is separately demonstrated for plants. The greatest amount of life can be supported by help of proportionally great diversification of struc ture; hence in small areas where competition is severe the inhabitants are extremely varied.
Sexual Not merely do individ uals struggle for existence, but the males strug gle for the females and the most vigorous tend to leave most progeny. Several weapons offen sive 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. Just as man can in a short time give beauty to his domestic birds, so there is no reason to doubt that female birds, in thousands of generations by selecting, as they are observed to do, the most melodious or beautiful males, might produce a marked result, and many sexual differences are thus explained.
Laws of The same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus. Specific characters are more variable than generic and varietal than either. Rudimen tary organs and secondary sexual characters are variable. Zebra-like stripes on horses or wood
pigeons' markings on fantails, tumblers, etc., may be explained as revisions toward their ancient progenitors.
Geological Succession of Organic Beings. — The most ancient forms differ widely from those now living, yet frequently present charac ters intermediate between groups now widely divergent and resemble the embryos of the more recent and more highly specialized animals be longing to the same classes. These and the important law of the succession of the same types within the same areas during the later geological periods, and most notably between the Tertiary Period and the present time, cease to be mysteries and become at once thoroughly intelligible on the principle of inheritance and on that alone. Darwin's belief that the distinct ness of birds from all other vertebrates was to be accounted for by the extinction of a long line of progenitors connecting them with rep, tiles, was in 1859 a mere assumption, but in 1862 the long-tailed and palpably reptilian bird Archaeopteryx was discovered, while in 1875 the researches of Marsh brought to light certain cretaceous birds, one with teeth set in a groove, the other with teeth in sockets and with bi concave vertebrae. Besides these reptilian birds, bird-like reptiles have similarly been forthcom ing, and the hypothesis of Darwin is thus ad mirably verified.
Geographical Neither the similarity or the dissimilarity of the inhabit ants of various regions, whether of land or 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 physiolog ical and distributional lines of argument fur nished by morphology are mainly four, and are derived from (a) Classification, (b) Homol ogies, (c) Embryology, (d) Rudimentary Or gans. 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 under sub-iamilies, families and orders, and all under a few grand classes. The nature of all these relationships established by naturalists re ceives a simple and natural explanation on the view of the common descent of allied forms with modification through variation and natural selection, no other explanation ever having been attempted. 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, independ ently of their habits of life, resemble one an other 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 cons strutted on the same pattern, bone correspond ing to bone. This conformity to type is °pow erfully suggestive of true relationship, of in heritance from a common ancestor); and ad mits of a simple explanation in terms of the evolutionary theory, thus strengthening that theory.