Various Modifying Agencies. 131 



by them will remain as a rudiment. The causes that brought about the 

 color of the skin in very many individuals, if not nations, certainly do 

 not now exist, and the diversity of colors is, in many cases, kept up by 

 the ancient inherited habit. 



Quick results in the color of the products of the epidermis are some- 

 times obtained in the lower animals by change of food. Thus, it is 

 stated by Semper, that the green of the feathers of the Brazilian Parrot 

 can be changed to yellow or red by feeding the bird on the fat of cer- 

 tain fishes; that the Bullfinch turns black when fed on hemp seed; and 

 that the colors of certain Canary birds, of the Indian bird Lori Kajah, 

 and of butterflies, depend on the quality of their food. There is nothing 

 at all incredible in this statement, in view of what we know of the ef- 

 fects of food in other things both directly and indirectly. It is well 

 known how bees develop their queen from the larva of what would 

 otherwise become a sterile female, by food of a peculiar sort. 



1 ' The larvae of a moth Acronycta entirely lose the habit of spin- 

 ning a cocoon before assuming the pupa state, when their food is insuf- 

 ficient, and both the pupa and moths are then smaller." Semper, in 

 experimenting with the Mexican Axolotl, an amphibian which is now 

 known to be the larval or incomplete stage of the Amblystoma, but 

 which can and constantly does reproduce in its larval state, kept eight 

 of them for two years without breeding by stinting them in their food. 

 He kept them in a small aquarium destitute of plants and sand. After- 

 ward be removed them to a large aquarium supplied with plants and 

 sand, pebbles, &c., and in the course of fifty hours they deposited about 

 1,000 eggs. This is another illustration of the fact that reproduction 

 is preceded by a check in general development, and is parallel to the 

 statement in regard to the frog, &c. , it answering the same end, 

 whether the pause in development is caused by defective temperature or 

 inadequate food. 



CHAPTER XX. 



VARIOUS MODIFYING AGENCIES. 



In the foregoing, many examples are given to show that changes in 

 animal organisms are caused by changes in their mode of life. Changes 

 in the mode of life, or habit, never can occur as long as the conditions 

 remain the same. The same stimulations must invariably produce the 

 same reactions. This is axiomatic in theory, and receives a myriad il- 

 lustrations every day. But if the stimulation changes it will beget 

 change in the reaction. 



Some further illustration of the effects of altered stimuli on organisms 



