tjS H. S. 'Jennings 



Of evolution in unicellular organisms. The question in the center 

 of interest is: How do new inherited characteristics arise? To 

 study this question a knowledge of the normal phenomena of vari- 

 ation and inheritance is required. Our first contributions will 

 therefore deal with these normal phenomena, with incidental 

 attacks on the main problem as opportunity presents. We shall 

 take up inheritance, variation, specific differences, correlation, 

 growth, regulation, selection, and related topics, dealing with 

 them by experimental, observational and statistical methods. A 

 large part of such a study, in the common infusorian Paramecium, 

 is now complete. The present instalment deals with the definite 

 and circumscribed problem of the fate of new structural characters. 



2 General Plan of the Investigation and Principles Guiding It 



In presenting the first instalment of an extensive series of inves- 

 tigations, it will be well to set forth in an introductory way the 

 general considerations which have guided the work, together with 

 its relations to previous investigations by the author. Though 

 apparently a complete departure from the matters dealt with in 

 most of my work up to this time, it is in reality a logical continu- 

 ation of my previous work. The latter has lain hitherto in the 

 field of the physiology of behavior and reactions. In this field I 

 have endeavored to analyze and isolate, so far as possible, the 

 various factors at work, keeping in the foreground of interest the 

 problem of how the behavior happens to be so largely adaptive. 

 It is possible to show that certain of the features of behavior — 

 and precisely certain adaptive features — arise during the life- 

 time of the individual, by physiological processes which appear 

 quite intelligible from a thoroughly causal standpoint. These 

 are the processes known variously as the formation of habits; as 

 learning; as modification by experience; as expressions of the 

 readier resolution of physiological states after repetition, etc. 



But in this field, as in all other parts of biology, we find many 

 characteristics, and particularly many adaptive features, which 

 have not arisen during the lifetime of the individual. Certain 

 structures, certain processes, certain reactions, often highly adap- 



