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THE AMERICAN NATURALIST [Vol. LVI 



of equilibrium, because then, so far as there is no unto- 

 ward interference from without, it would be natural to 

 think that the course of the process must be in a certain 

 sense a straight one, with a negative acceleration. Taken 

 literally, such a consideration is, however, purely specu- 

 lative and for the present, I think, a sterile speculation. 



Somewhat more clearly intelligible is a hypothesis 

 which arises from the study of hormones and their role 

 in development. It appears to be quite possible that the 

 effect of increase or decrease in the amount of a single 

 chemical substance in a species might be a complex 

 change in its structure, including modifications of size, 

 of the proportions of the different parts, of pigmenta- 

 tion, or of the other peculiarities which ordinarily arrest 

 the attention of students of evolution. This would be. 

 especially true if, instead of a change in the amount of a 

 hormone or other substance, it were a case of the forma- 

 tion of a new compound. Such changes, while directly 

 due to a single substance, might be greatly modified by 

 readjustments following the disturbances of the physio- 

 logical equilibria between the different parts of the body. 

 Compensatory readjustments of similar nature are, of 

 course, among the most familiar and interesting phenom- 

 ena in pathology. We are, accordingly, fully justified 

 in taking their possibility for granted. 



It is, therefore, conceivable that evolutionary changes 

 should be occasionally progressive and apparently ortho- 

 genetic, although due to a simple physico-chemical modi- 

 fication. No doubt, if it were desirable, such considera- 

 tions might be developed into a clear and possibly useful 

 theory of orthogenesis, but I am not qualified to do so. 

 My object is only to insist that changes which from a 

 morpliological standpoint are complex, continuous, and 

 progressive, may conceivably be due to a single, simple, 

 physico-chemical change. 



Such reflections, vague though they may be, clearly 

 point to a conclusion which is, I feel sure, inevitable for 

 the physical scientist ; morphological phenomena in them- 



