60 EVOLUTION 



of experiments will be found in H. De 

 Varigny's " Experimental Evolution " (1892), 

 and more recently in T. H. Morgan's " Experi- 

 mental Zoology " (1907); and we cannot here 

 do more than give a few typical illustrations. 



In a few cases it has been found possible 

 to induce experimentally what may be called 

 an adaptive response. Thus Professor Poul- 

 ton's beautiful experiments on the pupae of 

 certain butterflies show that the colour of 

 the chrysalid is affected by the colour of the 

 surroundings, which operates in some mys- 

 terious way through the skin. When the 

 pupation occurs on a light background the 

 chrysalids are lighter; when on a dark back- 

 ground the chrysalids are darker. This is 

 undoubtedly an advantageous response, for it 

 has been proved experimentally that in natural 

 conditions survival depends in great part on 

 the inconspicuousness of the pupae in the 

 place where they are fastened. 



In connection with experimentally induced 

 adaptive responses Professor T. H. Morgan 

 makes an important note : " It is remarkable 

 how rare are adaptive structural responses, 

 when we recall the fact that adaptation of 

 the organism to its surroundings is one of its 

 most characteristic properties. The poverty 

 of adaptive structural response does not 



