STAGES OF VITAL MOTION. 23 



mutations and hybrids. The weakness and sterility of too distant 

 crosses and of too closely isolated or inbred plants or animals may be 

 due alike to a deficiency of normal fertilization, and may be accepted 

 as evidence that the true course of evolution lies along neither of these 

 extremes but follows the natural mean between them. 



Disconiinucms Variation, 

 Catalytic variations have not the indefinite number and diversity of 

 the progressive stage; like the symptoms of other disorders of plants 

 and animals the same or closely similar mutations recur in somewhat 

 definite proportions, and are not peculiar to single species, but many 

 members of a genus or family may be similarly affected. It is there- 

 fore not necessary to interpret the independent repetition of the same 

 symptom of evolutionary debility as an evidence of the inheritance 

 of definite character complexes or units.* The truly admirable but 

 often misinterpreted experiments of Mendel did not result in the dis- 

 covery of 'principles of heredity' so much as they revealed limits of 

 hybridization, in that hybrid plants were found which inherited the 

 characters of only one parent. The failure of strongly divergent or 

 antagonistic characters to combine into a permanent average in hybrids 

 gives, however, no basis for denying that normal evolution proceeds by 

 the synthesis or accumulation of acceptable variations, nor is abrupt or 

 discontinuous variation in individuals in any way incompatible with the 

 probability that in nature evolution goes forward only tlirough the 

 gradual transformation and subdivision of species. The emphasis 

 placed by Bateson, De Vries and others upon abrupt variations is war- 

 ranted by no general pertinence of the facts, and is but a consequence 

 of the failure to perceive that the origination or multiplication of spe- 

 cies is an incident rather than an instance of evolution. 



Cross- fertilization Accelerates Evolution. 

 Organic succession will not persist on too narrow lines of descent, 

 does not normally leap aside from its course, and will not bridge over 

 too broad a chasm of evolutionary divergence. Amount of difference 

 in the external characters of two groups affords little indication regard- 

 ing the behavior of their hybrids. Some groups treated by the sys- 

 tematists as closely related species will not even hybridize, while in 

 other instances plants assigned to different genera are mutually fertile. 

 Such discrepancies are doubtless due partly to inadequate classification 

 and partly to the fact thal^ organic evolution is attended also by a 

 cytological or cellular evolution the progress of which may not be con- 



* Criminologists have found in the human species the same tendency of ab- 

 normal individuals to fall into recognizable types. Inbreeding is also recognized 

 as a frequent cause of aberrations from the mental and physical average of the 

 race, just as sterility and emotional abnormality are among the most frequent 

 phenomena of criminality. 



