972 LIFE : OUTLINES OF GENERAL BIOLOGY 



and no transitions are known between them and wings, yet their 

 evolutionary origin seems certain. 



Variations in the degree of development that wings attain are not 

 uncommon among insects; the facts suggest that the restriction of 

 the actual flying function to the second pair of wings has occurred 

 repeatedly, as in beetles, though the first pair may persist as useful 

 wing-covers. Many cases of secondary winglessness are known, as in 

 fleas, in contrast to the primary winglessness of the old-fashioned 

 Spring-tails (CoUembola) and Bristle-tails (Thysanura), in regard to 

 which there is no evidence that they ever had wings. We suppose, 

 then, that the extraordinarily perfect functioning of the fore- wings 

 in Diptera (sometimes striking the air two hundred times in a 

 second) made the hind-wings unnecessary as organs of flight. 

 Variations of a retrogressive character set in, and were rather 

 advantageous than otherwise; but when the reduction had gone 

 far, a rehabilitation of mobility and an acquirement of new sensitive- 

 ness came about, and so "poisers" evolved. 



(7) An instructive instance is afforded by the gill-filaments of the 

 tube-inhabiting Serpulid worms. On the head there are numerous 

 gill-filaments, which are exaggerations of the tactile "palps" of many 

 other Polychaet worms. They form a beautiful crown projecting at 

 the mouth of the calcareous tube and completely retractile when 

 the worm draws in its head. Now in many genera of Serpulids the 

 dorsalmost gill-filament, on one side or on both, is dilated at its end 

 into a club, sometimes homy, sometimes calcareous, sometimes 

 both, which closes the mouth of the tube when retraction takes 

 place. This stopper or operculum is very well seen in the common 

 Serpula, whose twisted tubes are often fixed to stones in inshore 

 waters. But in the tiny Spirorbis, which forms for itself a flat spiral 

 tube of lime on Fucus and on shells, the distal end of the operculum 

 is enlarged to form a brood-pouch for the eggs. In the Filigree 

 worm (Filigrana) the operculum is but slightly developed; in 

 Protula and Salmacina there is none. Thus we have a very inter- 

 esting series, of which Spirorbis is the climax. 



ATAVISM AND REVERSION 



The term "atavism" is commonly used in three senses: (i) It is 

 used to denote the hereditary reappearance of a character not seen 

 in the parents, or even in the immediate ancestry, but found in an 

 ancestral race, or in one related thereto. Thus markedly projecting 

 canine teeth in man have been regarded as re-expressions of a 

 Simian character, and supplementary mammae on the breast of a 

 woman have been regarded (probably quite erroneously) as atavistic 



