§ 3] CHEMOTAXIS 39 



the organism as food ; under which circumstances it can be 

 called "Trophotaxis."* In other cases it is a response to 

 chemical substances which have no significance as food, and 

 have no other importance for the organism. 



It is clear that we cannot assume that response to injurious 

 substances is an adaptation which has been brought about by- 

 natural selection. If a response occurs in one case independent 

 of the action of selection, we should hesitate to ascribe to this 

 cause the origin of other, even favorable, responses. 



General Remarks on the Relation between Molecular Composi- 

 tion and Response. — Pfeffbk ('88, pp. 608-612) has pointed 

 out that the capacity of any substance to stimulate cannot be 

 inferred from its chemical constitution and relationships. Thus, 

 the minimum concentration of milk sugar which will produce 

 a response is 1%, while in the case of the closely allied grape 

 sugar it is 10 % ; but in the very different kreatin it is also 1 % . 

 Also, the action of any chemical compound is determined not 

 by the elements, such as C, H, O, which it contains, but by the 

 entire molecule ; in other words, the atomic composition is less 

 important than the structure of the molecule or the arrange- 

 ment of its atoms. Thus, malic acid and its compounds with 

 neutral ammonium, sodium, barium, and calcium containing 

 0.001% parts of the acid, have an equal action upon the sperma- 

 tozoids of ferns, which do not react to the diethylester of 

 malic acid, even in strong solutions. (Pfeffer, '88, p. 655.) 

 Again, nitrogenous organic compounds are, in general, more 

 active than the non-nitrogenous ones ; but this cannot be held 

 to be due alone to the presence of N ; for dextrin (CgHj^Og) 

 is nearly as active as the nitrogenous peptone, and, on the other 

 hand, the nitrates of metals are not more active than their 

 chlorides, while the ammonia salts are relatively weak. 



Relation between the Strength of the Stimulus and that of the 

 Response. — When we say that malic acid attracts sperma- 

 tozoids we mean that under certain physical conditions of 

 the water which we may call normal it does so. And under 

 normal conditions of the water, it is only within certain limits 



* Stahl ('84, p. 164) called the attraction of Plasmodium of myxomycetes to 

 bark extract " Trophotropism." 



