PRESENT PROBLEMS 573 



school, of Pearson 1 and of Weldon, and which form the strongest re- 

 maining 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 varia- 

 tions 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 Saint-Hilaire 2 argued for pale- 

 ontological evolution by saltation, I do not think we have much evi- 

 dence in paleontology for the saltation theory. In the nature of the 

 case, we cannot 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 salta- 

 tions in the sudden variations in the number of vertebrae 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 

 cannot 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 sub- 

 sequently modeled by adaptive modification, as the bones, muscles, 

 and all the other tissues of the body are. Mutations are found arising 

 according to partly known influences of kinship. They do not, so far 

 as we observe, possess adaptive value when they first appear, but then 

 frequently, if not always, develop into a stage of usefulness. 



Fitness is, therefore, the central thought of modern paleontology 

 in its most comprehensive sense, as embracing fitness in the very re- 

 mote past, in its evolution toward the present, and in its tendencies for 

 the future. Just as the uniformitarian method of Lyell transformed 



1 K. Pearson. See articles in Proceedings of the Royal Society of London, and 

 in the Grammar of Science, London, 1900. 



2 Geoffroy Saint-Hilaire, Recherches sur des grands Sauriens trouvcs a I'etat 

 fossile, Mi-moires de 1'Academie des Sciences, Paris, 1831; Influence du monde 

 ambiant pour modifier les formes ammales, Memoires de 1' Academic des Sciences, 

 xii, 1833, p. 63. 



