74 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ential tolerance. It is often difficult to determine whether the modifica- 

 tions observed are due merely to the primary differences in tolerance or to 

 actual differential conditioning. Thus far. death gradients associated with 

 differential tolerance and differential conditioning have been investigated 

 in only a few forms and with a few agents, but difference in ability to toler- 

 ate and to acquire tolerance to different agents is evident. The differential 

 in tolerance to ethyl alcohol along the polar axis of forms tested is con- 

 siderable, and differential conditioning occurs relatively rapidly; but dif- 

 ferential tolerance to cyanide is shght, and differential conditioning occurs 

 ver>^ slowly if at all (Child, 1932&). It is possible that the mechanism of 

 tolerance to certain agents may gradually fail with time and that a pro- 

 gressive differential sensitization to certain agents may conceivably occur, 

 but evidence is lacking on these points. 



Living organisms are able to recover more or less completely from less 

 extreme toxic action of external agents after return to the normal medium. 

 Axial differentials in recovery after slight differential inhibition are, in gen- 

 eral, parallel to those of differential tolerance and differential acclimation 

 but are often more strongly marked, since the inhibiting agent is no longer 

 present in the recovery period. With more extreme action of the agent the 

 more susceptible regions may be injured to such an extent that they can- 

 not recover, even after return to normal environment, and only less sus- 

 ceptible regions recover. This is not, properly speaking, a differential re- 

 covery but rather a partial recovery, that is, a recover}- of the least sus- 

 ceptible and least inhibited part; usually it is by no means complete and 

 may be Httle more than a continuation of life after return to the natural 

 medium. The parts that do not recover usually die, though in some cases 

 they may remain alive in highly inhibited condition. 



Differential tolerance, conditioning, and recover}- are all secondary 

 modifications, as far as distinguishable alterations of pattern are con- 

 cerned. They depend on the physiological condition and metabolism of 

 the part concerned, rather than directly on the action of the external 

 agent. As regards both differential death and modification of form and 

 proportion in development, their effects are the reverse of the primary in- 

 hibitions in their relations to physiological pattern. 



As regards the physiological significance of differential susceptibility, 

 the fact that the lethal gradient is the same in direction for many different 

 agents, both chemical and physical, indicates that it does not depend on 

 the constitution or nature of the agent. It is certain that lethal effects of 

 different agents are not all brought about in the same way. The only con- 



