GENERAL CONCLUSIONS 271 



and integration on the grounds that, while various lines 

 of evidence indicate the existence of these gradients, we 

 do not yet know what they really are. As a matter of 

 fact, we do not know what anything in the organism 

 really is; we merely know more or less about it. Up 

 to the present we have learned many things about the 

 physiological gradients and many more remain to be 

 learned. We can state, however, on grounds much 

 more adequate than those on which many other scien- 

 tific conclusions rest, that these gradients are primarily 

 quantitative in character, that while they may persist 

 through reproduction, i.e., be inherited, they arise pri- 

 marily in relation to a quantitative differential in the 

 action of an external factor on protoplasm, that they 

 show all the characteristics of excitation-transmission 

 gradients, and, finally, that they are of fundamental 

 significance in the development and physiological inte- 

 gration of the organism. The evidence for the existence 

 of the physiological gradients and for their significance 

 is based largely on experimental data and is therefore 

 more definite and conclusive than much of the evidence 

 on which various other current biological theories have 

 been built up. 



While experimental data concerning many features 

 of nervous development are lacking, the main facts of 

 its relation to the gradients are clear. As regards many 

 points only suggestion, inference, or a weighing of proba- 

 bilities is at present possible, but it has seemed desirable 

 in the preceding chapters to follow through the line of 

 thought developing from the experimental foundation 

 and therefore to advance suggestions, to consider prob- 

 abilities, and to draw tentative conclusions in many 



