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THE AMERICAN NATURALIST [Vol. LIII 



their metabolism become so affected that they give rise 

 to lens tissue, instead of to iris tissue? Can we believe 

 that the iris cells proceeded unfalteringly to this end as 

 a result of the operation ? 



The discussion after all hinges upon the word '^unfal- 

 teringly," and this term has been applied to processes 

 which are beyond the possibility of direct observation. 

 If we grant that a disturbance of growth equilibrium was 

 what led to the reparative processes, and that equilib- 

 rium was in the end restored, it does not seem difficult 

 to admit that each minutest step in the direction of re- 

 storing this equilibrium was selected from a medley of 

 random reactions. Indeed, Holmes suggests that 



advantag-es which the symbiotic relation afforded the cells whose place 

 they take. Differentiation in any other direction deprives them of these 

 advantages and subjects them to other unfavorable conditions 



Nor need it be assumed that these responses are wholly 

 random. Although it is incredible that each type of 

 possible injury has been provided for in advance by a 

 specific mechanism, it seems more than possible that cer- 

 tain reactions have been acquired which are of service 

 in any emergency— a sort of "first aid to the injured," 

 as we might say. After these preliminary steps of a 

 general character— which are, as a matter of fact, the 

 common precursors of regeneration^®— the more special 

 processes may be supposed to proceed in a tentative 

 fashion. 



All that is meant by "growth equilibrium," in this dis- 

 cussion, is such a normal state of metabolic balance that 

 the growi;h of each part is checked through its organic 

 relations with the rest. Attainment of this goal would 

 bring the organism into a condition of "no stimulation," 

 like that of the protozoan which has escaped from an un- 

 favorable environment. 



Since we commonly are able to observe only the final 

 outcome of such a process, and overlook the minute steps 



