CHARACTERS OF SPECIES. 277 



sound distinctions for species, or at most — among 

 insects — for permanent or sexual differences. We re- 

 collect no instances among wild animals, in which these 

 marks are known to vary : no better or more tangible 

 character, indeed, can be named, than the relative shape 

 of the wings of birds, in cases where the size and colour 

 of two or more species are perfectly the same. A striking 

 instance of this may be seen in the greenlets {Vireo V.) 

 of America; where the different shape of the wings con- 

 stitutes the only specific distinction of three, if not of 

 four, species.* The possession of horns, protuberances^ 

 enlargements, &c. are mostly characters of types, but 

 their particular shape is a sure indication of species. 

 When, however, the shape and direction are the same, 

 but there is merely a difference of size or developement, 

 such circumstance cannot constitute a species. Let us 

 look, for examples of this, to the hornbills (^Bucerido!^ 

 among birds ; the Scarahceida- and the Cerambijcidce 

 among insects, and the Muricidce among shells. The 

 young hornbills have seldom any of those protuberances 

 on their bill, which they acquire with maturity; and 

 even then they increase in size, without altering much 

 in form, unto advanced age : from ignorance of this fact, 

 former writers were very apt to describe the young 

 and the adult bird as two different species. An analo- 

 gous case to this is met with among the saprophagous 

 beetles (particularly in the group of bulky Dynastidee 

 MacL., and in the genus Phcenius of the same author) : 

 the hornlike protuberances, which, in the last, distin- 

 guishes the male sex, vary, in their length, in almost 

 every individual ; so that in some they are very promi- 

 nent, while in others they are merely like short tubercles. 

 It would be curious to ascertain whether this difference 

 results likewise from age. The spines upon the different 

 rock shells (^Murex), and on the coronated volutes 

 (^Cymhiola Sw.), vary in like manner ; some specimens 

 having acute and prominent spines, while others are 

 nearly smooth. These are the most remarkable excep- 

 • See Northern Zoology. 

 T 3 



