DISEASES OF THE OX AND SHEEP. 429 



flowers, or stems, with which any given group of animals is most 

 intimately associated, and the group itself. Frequently, too, it 

 happens that a remarkable power of adaptive change is to be 

 noted ; as, for example, in the case of the chameleon, which 

 varies in colour according to its position in regard to external 

 objects and places. The protective resemblances, so generally 

 found among the class Insecta, are frequently of an extremely 

 marked type, and it becomes a matter of great difficulty to discern 

 the creatures, situated as they are in the natural state among the 

 objects to which they are so strangely assimilated in form, 

 colour, and general appearance. It is recorded of Mr. Wallace 

 that he was in the habit of placing in a box some butterflies, 

 appended, as they might perhaps be found naturally, to the 

 stalk of a shrub, and, together with those thus arranged, 

 another fully exposed to view. To his inquiry, '* How many 

 butterflies are there in that box?" the invariable reply of the 

 observer was that the case contained only one single specimen. 

 Then, after exposing one or two of those which appeared to be 

 leaves, he continued: "How many are there now?" The 

 reply was: "Oh ! I see that all the leaves are butterflies, and 

 hence it is only necessary to count the leaves in order to answer 

 your question." But the naturalist had his friends once more at 

 fault, for some were leaves and some were butterflies, although to- 

 the eyes of an unpractised observer all seemed identically similar 

 leaves. Not only, however, do animals simulate inanimate 

 objects and parts of plants in point of form and colour, but 

 other animals also are so exactly represented that only an 

 experienced naturalist can detect the superficial character of the 

 resemblance. In intimate structure the greatest possible dif- 

 ferences are to be observed. In some cases, insects simulate 

 the very animals which prey upon them, while, in other cases, 

 the wolf in sheep's clothing stands confessed when the would-be 

 devourer is seen to resemble closely the objects of his greed. 

 For instance, some spiders are very like their prey in size, form, 

 and colouring, and one is known which actually imitates the 

 actions of the poor fly, even in such a minute particular as that 

 rapid moving of the falces which is so well known and so 

 characteristic. Much stranger similarities, however, remain to 

 be noted. When the sexes of the animals which are mimicked 

 differ, the sexes of the mimickers sometimes also differ in a corre- 



