234 ANIMAL COLOEATION, 



of certain of the muscles used in respiration and in flight. 

 The absence of these in the Screamers allies those birds, which 

 in other respects do not occupy an especially low position in 

 their group, to the lower Eeptilia, which are believed to- 

 represent more nearly than any existing animals, the original 

 stock from which birds were evolved. 



With one exception all earthworms at present known to us 

 — and they comprise now a couple of hundred well-marked 

 species — possess setae, which serve as organs of locomotion,, 

 of a peculiar form, curved like the letter S and terminating in 

 a free, pointed extremity. In a very large number — by far the 

 majority — of the fresh- water members of this group of annelids 

 there are set^ of the same general form, which terminate in a 

 distinctly bifid extremity. One genus only of earthworm, 

 UrochcBta — a form which is widely distributed through the 

 tropics — possesses these bifid setfe. 



If colour markings have that importance which the advocates 

 of the theory of mimicry believe them to have, it is quite 

 intelligible that resemblances due to relationship may have- 

 survived in some members of a group and disappeared in 

 others. This would be analogous to arrested divergence in 

 unpalatable insects, to which reference has been made on a 

 former page; but it does not fit in with the defence of mimicry 

 by Mr. Belt, which I have quoted above. 



Even in cases of superficial resemblance between insects 

 belonging to diff'erent groups, the question of a possible affinity 

 must not be left out of sight. The Caddis flies mentioned on 

 p. 242 resemble Lepidoptera in their brown-coloured and hairy 

 body and wings ; these hairs may actually take the form of 

 scales. Some entomologists are of opinion that this resemblance- 

 is one of affinity.* 



* See Comstock, " An Introduction to Entomology," Part I. 



