NATURAL SCIENCES OP PHILADELPHIA. 27 



the insect to cany out its natural instincts, the cocoon of the same 

 species varies but little, if at all, from the characteristic type. 



It is well known that birds vary their styles of architecture in 

 correspondence with changes in their environment. What just 

 reason can be adduced for non-variation in the cocoon-making of 

 silk-spinners? Primitively, when the natural sequence of events 

 was not disarranged by the devices of man, a remarkable degree 

 of uniformity, doubtless, prevailed mnidifi 'cation among individuals 

 of the same species ; but, latterly, with some exceptions, however, 

 varieties do occur which betray such marked deviations from ordi- 

 nary types that to the most experienced and critical eye they seem 

 stamped with a newness of design truly astonishing. In the 

 absence of positive evidence per contra, they might, with some 

 show of reason, be attributed to the workmanship of essentially 

 different species of unknown habits. 



Sticklers for the doctrine of never-varying instinct as determin- 

 ing and controlling the actions of the feathered creation in oppo- 

 sition to intelligent reason, would easily persuade themselves, no 

 doubt, that perceptible differences did not occur, and would argue 

 that the same species manufactured the identical style of nest in 

 these latter times as in the beginning, in the face of the strongest 

 array of evidence. I 



Instances of variation have been recorded in ornithological 

 literature. The writer has noticed in the Proceedings of the 

 Philadelphia Academy of Natural Sciences, a remarkable devia- 

 tion fron the typical nest in the case of Sayornis fuscus, Baird, 

 so striking and marked as to elicit considerable astonishment, but 

 based upon the most satisfactory and positive evidence. 



If changes be introduced into the environment of a species of 

 avis, by the arts of civilized life or otherwise, sufficiently potent to 

 im [tress its sensorium, so as necessarily to lead to changes of habit 

 whereby a continuance of the species is provided for, and the 

 existing harmony of a moiety of creation remains undisturbed, 

 the argument is irresistible, that, in localities where the food-plant 

 or plants of a larva have disappeared through human civilization, 

 or defeat in the plant's "struggle for existence," being supplanted 

 by another better adapted to the new conditions of life, the insect 

 itself will either succumb or adapt itself to the altered phase of 

 affairs. 



Such facts as this hypothetical case presupposes, have fallen 



