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THE AMERICAN NATURALIST [Vol. XLVIII 



I. INTRODUCTION 



Duking the past few years the attention of biologists 

 has turned more and more from those phenomena which 

 were supposed to be comparatively fixed, to responses to 

 stimuli. Physiologists have long been concerned with 

 the mechanism of response ; psychologists are interested 

 in its modification. Geographers, climatologists and 

 ecologists have recently turned their attention to re- 

 sponses in natural environments and zoologists have 

 become interested in response, particularly from the point 

 of view of its specificity. In these quite independent 

 investigations and compilations there has been little 

 attempt at analysis with a view to determine legitimate 

 lines of comparison among the exceedingly diversified 

 types of organisms which have been investigated, and 

 some confusion has resulted. For example, since the 

 more obvious responses of plants are structural, persons 

 not familiar with comparable phenomena among animals 

 have made erroneous comparisons of sessile plants and 

 motile animals. This paper is written to present in as 

 nearly uniform terms as practicable (a) analysis of kinds 

 or aspects of response, (b) justifiable kinds of compari- 

 son, and (c) the bearing of response phenomena on 

 biological theory and controversy. It aims to show that 

 the numerous kinds of response are reducible to a few 

 simple types common to both plants and animals, and that 

 the failure to consider all types has been responsible for 

 confusion and various one sided theories. It further aims 

 to show that study of response during the past few years 

 has led to an unusual broadening of our conceptions. 



EL BASTS OF DISCUSSION 



As a basis for discussion we must first have a clear 

 understanding of the character and definition of response. 

 Secondly, we must determine what constitutes an indi- 

 vidual in those plants and animals that are made up of 

 repetitions of parts. Thirdly, we must note whether or 

 not the organism is sessile or motile, capable of playing 

 the part of either, or colonial pelagic. 



