GENERAL FEATURES 31 



able. If it were necessary for one it would be so for all. On the 

 contrary, butterflies are generally distinctly and brightly colored, 

 and in their habits they delight to flaunt their glittering wings in 

 the face of the world. They fear nothing; and if attacked they 

 know how to escape. 



As to birds, note that they are in perpetual fear for every mo- 

 ment of their waking lives ; not for one instant are they free from 

 caution as against their enemies ; it seems as if they wanted and 

 needed eyes that should look backwards and sideways as well as 

 forwards, for they are always trying to see in every direction at 

 once; why, then, are not birds protected by mimicry, for they 

 really seem to need it ; but butterflies do not need it. . 



§ 31. Movement of Butterflies. 



By "movement" I mean the substitution of the abnormal female 

 for the normal ; the temporary or permanent disappearance of an 

 entire species; and the unaccountable appearance of lost or un- 

 known species ; in fine, the change from one state of things to an- 

 other state. 



Where, now, are the normal-colored females of Argynnis Nito- 

 cris, Nokomis, Letof They are not in existence anywhere, but, 

 occupying their places are dimorphic or melanic females, wholly 

 dissimilar from what the normal females should be. 



Observe Colias Eurytheme, and all of those species of Colias 

 that have an albino female. Like the males of the Argynnids just 

 mentioned the males are constant, there is no dimorphism in that 

 sex, but there are two females existing, the normal yellow, and 

 the white or albinic ; the latter increasing rapidly in proportion of 

 numbers as the years go by, until apparently it will be but a few 

 decades, or a few hundred years at most, when those species of 

 Calias will have no normal yellow colored females in existence. 

 Twenty-five years ago in California the albinic females of Eury- 

 theme were extremely rare ; but today they are common. The 

 same report also comes from Colorado. It has been suggested 

 that perhaps the increasing growth and use of alfalfa, (Medicago 

 sativa, a plant not a native of America but introduced from 

 Europe,) upon which plant the larvae of Colias now chiefly feed, 

 but upon which in former times they could not feed, is, to a greater 

 or less extent, the cause of the dimorphic change ; but I do not see 

 any weight in this suggestion. 



