140 THE AMERICAN NATURALIST [Vol. LVI 



It lias been demonstrated experimentally that the limb 

 proportions in the brief life of a dog may be modified 

 from the cursorial to the saltatorial type by amputating 

 the fore limb. This is a process of reciprocal Modifica- 

 tion and Selection which Osbom, Baldwin, and Morgan 

 term Organic or Coincident Selection. I have devoted 

 an immense amount of study to the causes of the evolu- 

 tion of proportion and have come to the conclusion that 

 orthogenesis in the evolution of proportion may be ap- 

 parent rather than real. In other words, whenever a 

 character assumes a survival or elimination value, it may 

 develop very rapidly through the selection of fluctua- 

 tions in the right direction and may result in apparent 

 but not real orthogenesis. 



4. Summary as to Orthogenesis 

 The visible mutation of Waagen," or ''definite vari- 

 ation" or "rectigradation" of Osbom appears to de- 

 pend on the "Mutationsrichtung" in the germ-plasm. 

 The final question in my mind, as in yours, must be, if 

 such a "MutationsricJitung" exists, is it the "internal 

 perfecting tendency," is it the "vitalism," is it the 

 "creative evolution" which the majority of biologists 

 are so skeptical about? 



I observe that it is not. T observe that while the "Mu- 

 tationsrichttmg" is a real process, it differs from any 

 kind of internal perfecting tendency in the fact that it 

 consists in an adaptive reaction to the particular environ- 

 ment in which a series of organisms is placed, or to 

 which it migrates. For example, the internal perfect- 

 ing tendency to arboreal life does not manifest itself 

 when the animal seeks an aquatic life. Conversely, 

 aquatic adaptations are not constantly springing up 

 among arboreal mammals. Observations on fossil forms 

 have led to Dollo's remarkable generalization regarding 

 "alternate adaptation," which renders any form of in- 

 ternal perfecting tendency in any predetermined direc- 

 tion inadmissible. 



