222 THE PROBLEMS OF EVOLUTION 



be called upon in the natural state to adjust them- 

 selves many times to fluctuations of the environ- 

 ment, so we must avoid this condition as far as 

 possible. 



The behavior of the oflFspring of animals kept 

 for several generations under unusual conditions 

 has varied in the work of different investigators. 

 It is unfortunate that so many of these experi- 

 ments have been conducted without adequate 

 checks, as, for example, Schroder's studies of the 

 habits of the willow moth,^ for while any room for 

 objection is left by the experimenter he may be 

 certain that it will be forthcoming. Nevertheless, 

 the available experimental data strongly support 

 the points suggested above. The permanence of 

 characters is in proportion to the length of treat- 

 ment in some cases, and a return to normal within 

 one or two generations of the renewal of normal 

 environment is a common result. Apparent per- 

 sistence of a character is reported by Guyer and 

 Smith in their work with cytolysins,^ but, without 

 denying the positive value of these results, I feel 

 that normal responses to artificially modified ex- 

 ternal environments are the most desirable mate- 

 rials for this work. 



The theoretical possibilities of individual re- 

 sponses have not been adequately tested by experi- 



5 Verh. d. Zool. Ges., Vol. XIII, p. 158, 1903. 



6 Jo. Exy. Zool, Vol. XXXI, p. 171, 1920. 



