THE COLOURS OF ANIMALS AND PLANTS 121 



The effect of environment in causing changes of 

 colour is well shown in cases of what is known as 

 seasonal dimorphism, where animals produce two 

 broods in each year, each of different appearance 

 with regard to colouring, and each capable of 

 producing the other. A good example of this is 

 found in the two continental butterflies J T anessa 

 prorsa and Vanessa levana. 



Vanessa levana, the spring form, has a red ground 

 colour, with black spots and dashes, and a row of blue 

 spots round the margin of the hind wings ; Vanessa 

 prorsa, the summer form, is deep black, with a broad 

 yellowish-white band across both wings, and with no 

 blue spots. These were formerly called distinct 

 species, but have recently been shown to be varieties 

 of one and the same species. That these differences 

 are due to the direct action of cold and heat has 

 been shown by keeping the pupae of levana at a low 

 temperature. These, which would ordinarily have 

 produced the summer form prorsa, hatched, under 

 the altered conditions of temperature, partly as 

 levana and partly as an intermediate form. 



Again, very young Canaries change their colour to 

 orange when given Cayenne pepper, and certain 

 parrots have been shown to change their colour 

 when fed on the fat of a particular fish. 



It is very difficult to draw the line between the 

 direct action of environment through the nervous 

 system, and the action of Natural Selection ; for to 

 which can we attribute the whitening of Arctic 

 animals ? 



We have now to consider the great mass of cases 



