

232 POPULAR SCIENCE MONTHLY. 



degree beyond the parental form ; the wonderful phenomena of corre- 

 lated development which puzzled Spencer so much are chiefly attribu- 

 table 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 Quete- 

 let'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 paleonto- 

 logical 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 paleon- 

 tologist 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 abbre- 

 viation 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 sub- 

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



