Sigh school zoologV. 28? 



going reduction, and before all trace of them disappears, they 

 are styled " rudimentary" organs ; such are the rudimentary 

 metapodials of the horse. Organs like these receive their only 

 satisfactory interpretation when we look at them in the light of 

 the doctrine of descent with modification. 



26. The active relations between different species discussed 

 above, lead us to the consideration of the remarkable phenomena 

 of protective resemblance and mimicry (VII., 26, 33) — pheno- 

 mena which api)ear to be explainable only by natural selection. 

 The word mimicry conveys a striving after similarity, which is 

 entirely at variance with such an explanation, and the term is 

 now really taken to mean the preservation by Nature of 

 variations in the du-ection of i*esemblance to some other animal, 

 protected either by offensive or defensive . weapons. Cases of 

 such protection are recorded above. A. further illustration is 

 afforded by certain South American butterflies belonging to the 

 family Pieridse, which " mimic" those of another family, the 

 Heliconidfe, protected from insectivorous birds by their offensive 

 odour and taste. 



Protection may also be secured, however, by resemblance in 

 colour, or form, or both, to surrounding plants or inanimate 

 objects. The winter white coat of Arctic animaLs, the coloure 

 of the nests and eggs of birds, the form of the leaf- and walking- 

 stick insects (VI [, 23), are all to be explained in the same way. 

 Occasionally such likenesses are employed, not for defensive, 

 but for offensive purposes. One species of the predatory genus 

 Mantis (a member of a family allied to the Phasmidse) re- 

 sembles an orchid which is visited for its honey by bees, and a 

 species of spider has been observed, which, in the attitude in 

 which it waits for its prey, has the innocent appearance in 

 colour and form of a bird-dropping ! 



Colouration, in fact, is very generally protective in its func- 

 tion, and the transparency of most pelagic animals, the change- 



