EVOLUTION OF THE COLORS OF BIRDS. 7 



physiological units in the following terms :"* " If, then, 

 this organic polarity can be possessed neither by the 

 chemical units nor the morphological units, we must 

 conceive it as possessed by certain intermediate units, 

 which we may term physiological. There seems no alter- 

 native but to suppose, that the chemical units combine 

 into units immensely more complex than themselves, 

 complex as they are; and that in each organism, the 

 physiological units produced by this further compound- 

 ing of highly compound atoms, have a more or less dis- 

 tinctive character." 



Before considering the subject of heredity in further 

 detail, it may be well to pause a moment to consider its 

 bearing upon the question in hand the inheritance of 

 acquired characters. Obviously the nature of the me- 

 chanical process by which heredity is made possible 

 must most decisively determine what the possibilities of 

 heredity are just what characters can be inherited, and 

 what characters cannot (if any such exist). Darwin be- 

 gan with the assumption that all characters could be in- 

 herited and framed his theory of heredity upon this 

 assumption. In this he was followed by the various 

 subsequent writers on the subject, with the exception of 

 Spencer, whose theory does not appear to be designed 

 with the express view of accounting for the inheritance 

 of acquired characters, but rather to have been con- 

 structed inductively. 



Such was the state of the case when, in 1885, Mr. A. 

 E. Shipley in an article in " The Nineteenth Century," 

 called the attention of English and American scientists 

 to the views of Prof. August Weismami, of Freiburg. 

 Since then two editions of an English translation of the 

 collected essays of Prof. Weismann on the subject of 



* Principles of Biology, I, 183. 



