94 



BIOLOGICAL LECTURES, 



tions often exhibit a strong capacity for inheritance. Bateson 

 reaches in the conclusion of his work a modified form of St. 

 Hilaire's factor of saltatory evolution, and believes that species 

 have largely originated by 'discontinuity' of variation or the 

 sudden accession of new characters from unknown causes, con- 

 cluding that all inquiry into the causes of variation is pre- 

 mature. The materials he has brought together are of the 

 greatest value, and he has already been able to throw in doubt 

 many current beliefs, such as that variability is greater in 

 domestic than in wild animals. His interpretation of these 

 materials is, as we have seen, weakened, so far as it bears on 

 our search for the evolution factors, by the fact that from 

 the nature of most of his evidence he cannot discriminate 

 between ontogenic and phylogenic variation : moreover, he 

 discards any attempt to discriminate between palingenic and 

 cenogenic variations. This lack of analysis leads him into 

 what appears to be an entirely erroneous induction, for the 

 principle of discontinuity is opposed by strong evidence for 

 continuous and definite phylogenic variation as observed in 

 actual phyletic series. 



NAGELI'S FACTOR AND PHYLOGENIC VARIATION. 



Nageli's factor 1 introduces us to an entirely distinct territory 

 to the opposite extreme from saltation. It is one we can no 

 longer set aside as transcendental because of the strong like- 

 ness it bears at first sight to the internal perfecting principle 

 of Aristotle. It is supported in a guarded manner by Kolliker 

 and Ziegler. It contains the large element of truth that the 

 trend of variation and hence of evolution is predestined by the 

 constitution of the organism ; that is, granted a certain heredi- 

 tary constitution and an environment favoring its development, 

 this development will exhibit certain definite directions, which 

 when reaching a survival value will be acted upon by selection. 

 I have recently 2 described as the ' potential of similar varia- 



1 C. v. Nageli: Mechanisch-physiologische Theorie der Abstammungslehre. 

 Miinchen und Leipzig, 1884. 



2 Rise of the Mammalia in North America. Stud. BioL Dept. Columbia Col- 

 lege, vol. I, No. 2, September, 1893. 



