THE FACTORS OF EVOLUTION 191 



animalcule lies quietly, rolled up into a ball, but soon it pro- 

 trudes in all directions short amorphous pseudopodia, and we see 

 before us a form which reminds us of a young A. proteus. Then 

 the whole protoplasmic body expands longitudinally, and the 

 typical A. Umax crawls about beneath our eye. If we now leave 

 the animalcules undisturbed they will permanently retain this 

 form, but if we make the w r ater slightly alkaline by adding a trace 

 of liquor potassae the Amoebae suddenly roll themselves up 

 as if frightened, but gradually become accustomed to these 

 changed conditions and project once more their pseudopodia. 

 But remarkable to relate, these are now no longer broad and 

 shapeless, but pointed and spiny : the Amczba Umax has become 

 a typical radiosa. This form remains permanent as long as the 

 water remains slightly alkaline; but when they are once more 

 placed in ordinary water the compulsory form disappears and the 

 normal Amoeba Umax reappears. 



Among higher animals and vegetables numerous instances 

 are known which form an eloquent proof of the dependence of 

 the organisms upon their environment. We have already seen 

 that the temperature exercises a far-reaching influence upon the 

 colour of the butterflies. Phenomena such as have been obtained 

 artificially in the Tiger-moth and other butterflies may be also 

 observed in Nature and are then described as seasonal dimorphism. 



A little butterfly (Vanessa levana) occurs with us in two 

 forms which differ so greatly from each other that for a long 

 time they were regarded as two distinct species. As soon as 

 the sun sends down its first warm rays the little spring-form 

 emerges from the pupa and begins its flight. Each collector 

 knows the pretty brown-coloured butterfly with the coloured 

 design on its wings. From the eggs of the spring-form deve- 

 lops about July the summer-form, which is not only considerably 

 larger but possesses a black ground colour with light spots and 

 stripes. As the pupae of the spring-form must pass through the 

 cold of winter, but as the summer form is never exposed to cold 

 the attempt was made to seek an explanation of the difference 

 in appearance in the temperature. Experiments confirmed the 

 hypothesis : we may at any time produce the spring-form from 

 pupae which under normal conditions would have been developed 

 in the summer-form simply by exposing them to cold. Con- 



