PREFACE ix 



nomenon of response to external stimuli. Again it is with 

 living organisms that we have to deal, and, accordingly, no 

 distinction should be made between animals and plants. I 

 have, indeed, made no such distinction ; nevertheless, tastes 

 and training have led me to lay especial stress upon animals. 

 Even this is unfortunate, for the problem with which we are 

 concerned is precisely the same problem in all living organisms. 



In the second place, much stress is laid upon the quantitative 

 measurement of agents and effects. The lack of precision in 

 many investigations can hardly be too strongly decried ; for it 

 often results in confusion and useless disputes. On the other 

 hand, there is good reason for believing that exact measure- 

 ment is the key to many of the most puzzling of our problems, 

 and important results are to be expected from its use. 



As for the aim of the book, it is twofold. I have hoped on 

 the one hand that it might be readable to those who are inter- 

 ested in the general matters of which it treats matters of 

 importance for philosophy, for psychology, and for pedagogy 

 For man is an organism, and the development of his qualities 

 is modified by just those agents which guide the development 

 of other organisms. My primary aim, however, has been a dif- 

 ferent one. It is this aim to which other purposes have been 

 made subservient, which justifies the historical treatment that 

 has been often adopted, and justifies also the detailed descrip- 

 tions of methods which the lay reader will, naturally, omit. 

 This aim is so to exhibit our present knowledge in the field 

 of experimental morphology as to indicate the directions for 

 further research. 



A few words of explanation and acknowledgment are neces- 

 sary : It was planned at the first to issue all four parts of the 

 work at once ; but the task grew in the doing, while the need 

 of its publication became more pressing. So it was decided 

 to issue the work in parts as soon as each should be done. 

 Even under this arrangement it has not been possible to 

 include some of the papers of the last six months ; especially 

 I regret the omission of important papers by VERWOEN and 

 LOEB upon Galvanotaxis. In writing a book of this sort, 

 which draws upon several sciences, I have had recourse to the 

 kind assistance of several of my colleagues in the physical and 



