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THE AMERICAN NATURALIST [Vol. XLII 



ditions. Three sets of phenomena we have observed 

 which, though usually treated in the category of stimula- 

 tion, draw a clearer interpretation from the conception of 

 reaction-velocity. These were: (1) the relation of devel- 

 opment to the absence or deficit of single essential food 

 constituents; (2) the occasional striking effect of minute, 

 traces of added foreign substances upon the whole rate 

 of growth and metabolism; and (3) the general doubling 

 of the activity of vital processes by a rise of 10° C. 



The next higher stratum of principles should be the 

 complications introduced by limiting factors which inter- 

 rupt the extent of the manifestations of these principles 

 and by various correlations, as, for example, that by which 

 the reaction-velocity of one catabolic process might with- 

 draw the supply of material needed for full activity of 

 another different process. To this sort of relation may 

 be attributed that phenomenon so characteristic of the 

 more complex vital processes and quite unknown in the 

 inorganic world, namely, the optimum. 



Finally, superposed upon all this comes the first cate- 

 gory of phenomena that we are content still to regard as 

 stimulatory. From the point of view of metabolism and 

 reaction-velocity many of these appear very trivial, 

 though their biological importance may be immense. 

 Think how little the tropistic curvatures of stems and 

 roots affect our quantitative survey; yet a little rear- 

 rangement of the distribution of growth on the two sides 

 of an organ may make the difference between success and 

 failure, between life and death. 



From our present point of view vision does not extend 

 to the misty conceptions of stimulation upon our horizon. 

 We may therefore postpone speculation upon the mechan- 

 ical principles governing them and await the time when 

 by scientific operations we shall have reduced to law and 

 order the intervening region, which we may entitle the 

 chemical substratum of life. This done we may ven- 

 ture to pitch our laboratory a march nearer to the phe- 

 nomena of protoplasmic irritability and make direct at- 

 tack upon this dominating conception, the first formidable 

 bulwark of vital territory. 



