222 UNIVERSITY OF MISSOURI BULLETIN 



ent purpose, it will be sufficient, if we group these criticisms 

 under the following three heads : 



1. Criticisms based on the argument that natural selec- 

 tion unaided cannot account for the incipient stages in species- 

 formation. 



2. Criticisms based on the contention that accidental or 

 indiscriminate variations cannot give rise to adaptive changes, 

 as maintained by Darwin, and hence the necessity of assum- 

 ing the existence of determinate variations appearing in 

 straight, definite lines. 



3. Criticisms based on evidence presented from actual 

 observation and experimentation to prove that the slight fluc- 

 tuating variations emphasized by Darwin do not lead to per- 

 manent change, but that those variations which are instru- 

 mental in forming new species are of the definite or discon- 

 tinuous type, known as mutations. These criticisms have led 

 to the suggestion of a number of theories, designed either as 

 supplements to or substitutes for natural selection, the most 

 important of which I shall briefly refer to. 



Criticisms of the First Group (Incipient Stages). — Early 

 in the history of Darwinism, the objection was raised, and it 

 has since been often pressed, that until a variation has become 

 well marked it cannot be useful, and hence cannot determine 

 survival ; and, furthermore, if left merely to natural selection, 

 a favorable variation in its incipient stages would soon dis- 

 appear in the descendants of the organism showing that varia- 

 tion through the swamping efifects of cross breeding with 

 individuals lacking it. The difficulty, however, might be avoided, 

 if we could assume some means of preventing interbreeding 

 when the variation first appears, and this mode of escape has 

 led to the formulation of certain theories of isolation or seg- 

 regation, according to which those individuals in which the 

 variation has arisen would become separated, either by physi- 

 cal barriers or by physiological incompatibility, from the rest 

 of the species and thereby allowed to breed together. Such 



