Page:Popular Science Monthly Volume 66.djvu/236

232 degree beyond the parental form; the wonderful phenomena of correlated development which puzzled Spencer so much are chiefly attributable to this principle.

These adaptive modifications are not directly inherited, as Lamarck

(supposed, but acting through long periods of time there results the organic selection (Morgan, Baldwin, Osborn), of those individuals in which hereditary predisposition happens most closely to coincide with adaptive modification, and there thus finally comes about an apparent, but not real, inheritance of acquired characters, as Lamarck, Spencer and Cope supposed.

3. Variations of Degree.—We should by no means exclude as true causes of evolution associated with both the above factors, the selection of those variations of degree or around a mean which conform to Quetelet's curve, the subject of the chief investigations of the Galton school, of Pearson and of Weldon, and which form the strongest remaining ground for Darwin's theory of selection in connection with fortuitous variation. For example, I regard the appearance of long-necked giraffes, of slender-limbed ruminants and horses, of long-snouted aquatic vertebrates, as instances of the selection of variations around a mean rather than of the selection of saltations. The selection of such variations where they happen to be adaptive has been an incessant cause of evolution.

4. Saltation.—Although Geoffroy St. Hilaire argued for paleontological evolution by saltation, I do not think we have much evidence in paleontology for the saltation theory. In the nature of the case, we can not expect to recognize such evidence even where it may exist, because wherever a new form appears or a new character arises, as it were, suddenly, we must suspect that this appearance is due to absence of the connecting transitional links to an older form. The whole tendency of paleontological discovery is to resolve what are apparently saltations or discontinuities into processes of continuous change. This, however, by no means precludes saltation from being a vera causa in past time, as rising from 'unknown' causes in the germ cells and as forming the materials from which nature may select the saltations which are adaptive from those which are inadaptive. The paleontologist has every reason to believe that he finds saltations in the sudden variations in the number of vertebra? of the neck, of the back, of the sacral region, for example. In the many familiar cases of the abbreviation or elongation of the vertebral column in adaptation to certain habits, a vertebra in the middle of a series can not dwindle out of existence, it must suddenly drop out or suddenly appear.

5. Mutation.—These new characters are also germinal in origin, because they appear in the teeth, which are structures fully formed beneath the surface before they pierce the gum, and therefore not subsequently modeled by adaptive modification, as the bones, muscles and