COLOR IN ANIMALS 145 



Voyage de r Astrolabe, the oriole of Bourru is actually 

 described as a honey-sucker, and Mr. Forbes tells us that, 

 when his birds were submitted to Dr. Sclater for description, 

 the orioles and the honey-suckers were, previous to close 

 examination, considered to be the same species." 



Well-authenticated examples of mimicry among mam- 

 mals, or other vertebrates than the birds and reptiles, are 

 not numerous. Among the invertebrates, outside the classes 

 ;>f the insects and the spiders, there are some instances 

 known, but as they are not very frequent, and, as they are 

 seen in forms which are less generally known, we will not 

 refer to them. 



Wallace mentions five conditions which are always ful- 

 filled in cases of mimicry. Let me quote his statement 

 of these. 



" i. The imitative species must occur in the same area 

 and occupy the very same station as the imitated. 

 " 2. The imitators are always the more defenceless. 

 " 3. The imitators are always less numerous in individuals. 

 " 4. The imitators differ from the bulk of their allies. 

 " 5. The imitation, however minute, is external and visible 

 only, never extending to internal characters or to 

 such as do not affect the external appearance." 



The instances thus far mentioned are all of protective 

 mimicry. Of aggressive mimicry there are but very few 

 instances known. Some of the hunting spiders are very 

 like the flies on which they prey; possibly also the ant- 

 like spiders can more readily approach their prey because 

 of their resemblance to ants which may not be so much 

 avoided by small flies (Figs. 38 and 39). Certain insects, 



