2 5 o THE POPULAR SCIENCE MONTHLY 



complex, and it would seem as though the system would soon fall of its 

 own weight. The entire value of the hypothesis consists in the formal 

 approximate expression of certain facts; when it is found that the 

 hypothesis begins to fail even for the classes of facts for which it was 

 originally intended, and that most of the known facts of development 

 can not possibly be expressed in its terms, the entire conception is put 

 on trial. 



The weakness in the theory of unit characters is in the use and 

 conception of the term " character.' The term has been prescribed 

 to us by the systematic zoologists and botanists engaged in describing 

 the differences between species ; so that " character " really means any 

 definable feature of an anatomical kind that differentiates species; by 

 extension it also means any other differentiating features that can be 

 defined. In the study of evolution and heredity, it is usually only 

 anatomical characters that are in question. Now the study of the 

 physiology of development teaches us that whatever else " characters " 

 may be, they are not units; they simply represent the sum of all 

 physiological processes coming to expression in definable areas or ways, 

 and they may thus represent a particular stage of a chemical process, 

 or a mode of reaction of some part. " Character " is essentially a 

 static morphological term; in the study of heredity and development 

 we are dealing with biological processes. To adapt a phrase of Hux- 

 ley's : " characters " are like shells cast up on the beach by the ebb and 

 flow of the vital tides; they have a more or less adventitious quality. 

 To give them representation in the germ is equivalent to a denial of 

 uniformity in biological phenomena. 



Just as the exact position of each shell on a beach might be fully 

 explained if we knew its full history, so each character has a certain 

 kind of inner necessity as the result of a sequence of developmental 

 processes. And just as in the history of the position of the shell on 

 the beach we should certainly ascribe great importance to the tides and 

 winds, so in the quality of each individual character we should find 

 corresponding vital tides and winds, as regular and lawful as those of 

 the ocean. We do not yet know the secrets of the vital tides; we 

 maintain only that they are the moving forces in development and 

 heredity, just as in physiology and pathology; and every fundamental 

 contribution to the physiology of protoplasm is at the same time, 

 and to the same extent, a contribution to heredity and the physiology 

 of development. 



But if these principles are accepted, how are we to explain the 

 facts on which the theory of unit characters depends? The main 

 difficulty lies not in the facts of mutation, for the physiology of this 

 phenomenon already begins to appear from the experiments of Tower 

 and MacDougal, who show that mutations may result from action of 



