Vin PREFACE 



morphological structure of nervous character. This 

 book is first of all a consideration in the light of 

 recent experimental investigation of certain of the 

 physiological conditions which antedate the appearance 

 of the nervous system, and with which its appearance 

 and development appear to be closely associated. It is, 

 in fact, an attempt to establish the existence of physio- 

 logical continuity between the simple quantitative 

 gradient in physiological condition and the nervous 

 system. As I shall endeavor to show on the basis of 

 experimental data, the physiological gradient arises in 

 the final analysis as an excitation-transmission relation 

 in response to the differential action of an external factor 

 and at the same time represents the primary integrating 

 factor of the sort characteristic of the organism. The 

 nervous system represents the organ of integration par 

 excellence, and some sort of continuity must exist 

 between the primary factors concerned in determining 

 those relations between regions or parts which make the 

 organism an orderly whole and the nervous system, 

 which continues the development of this integration to 

 its highest point. Since the book is concerned first 

 with this question of continuity and its nature and since 

 no general consideration of the physiological gradients 

 has appeared since 191 5, it has been found necessary to 

 devote the earlier chapters to a discussion of pattern in 

 the organism and its relation to the gradients. These 

 chapters constitute the basis for the consideration of the 

 nervous system in the stricter sense, which follows. 

 Believing as I do that the conception of the physiological 

 gradient enables us to throw some light, not only on the 

 problem of development in general, but particularly on 



