198 TROPICAL NATURE, AND OTHER ESSAYS. 



Until tlie habits of the zebra have been observed with 

 special reference to these points, it is surely somewhat 

 hasty to declare that the stripes "cannot afford any 

 protection/^ 



Colour Proportionate to Integumentary Development. 

 — The wonderful display and endless variety of colour 

 in which butterflies and birds so far exceed all other 

 animals, seems primarily due to the excessive develop- 

 ment and endless variations of the integumentary struc- 

 tures. No insects have such widely-expanded wings in 

 proportion to their bodies as butterflies and moths ; in 

 none do the wings vary so much in size and form, and 

 in none are they clothed with such a beautiful and 

 highly-organized coating of scales. According to the 

 general principles of the production of colour already 

 explained, these long-continued expansions of membranes 

 and developments of surface structures, must have led 

 to numerous colour-changes ; which have been sometimes 

 checked, sometimes fixed and utilised, sometimes inten- 

 sified, by natural selection, according to the needs of the 

 animal. In birds, too, we have the wonderful clothing 

 of plumage — the most highly organized, the most varied, 

 and the most expanded of all dermal appendages. The 

 endless processes of growth and change during the deve- 

 lopment of feathers, and the enormous .extent of this 

 delicately- organized surface, must have been highly 

 favourable to the production of varied colour-effects ; 

 which, when not injurious, have been merely fixed for 

 purposes of specific identification, but have often been 

 modified or suppressed whenever difi"erent tints were 

 needed for purposes of protection. 



Selection by Females not a Cause of Colour. — To 



