i6o 



SCIENCE. 



[Vol. XXI. No. 529 



SCIENCE; 



Published by N. D. C. HODGES, 874 Broadway, New York. 



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THE EVOLUTION AND USE OF THE AFTERSHAFT IN 

 BIRDS. 



BY HUBERT LYMAN CLAKK, PITTSBURGH, PA. 



The presence of an aftersliatt on the contour-feathers of the 

 body has long been recognized as a taxonomic character of some 

 value in the classification of birds, but little, if anything, has 

 been published regarding its history or use Admitting that every 

 part of the body either has some function, the exercise of vrhich 

 has tended to preserve and strengthen it, or else that, if function- 

 less, it is gradually being atrophied, it is necessary, before the 

 evolution of any organ can be followed through all its stages, to 

 first discover if it has any function, and, if so, what it is. The 

 probable history of the aftershaft can best be traced in this way, 

 and so the first point to be settle.^ is the question of its use. The 

 primary function of feathers is the retaining of heat, and, while 

 forming a non-conducting covering, it is still essential that the 

 weight be as little as possible. As the featheis differentiated 

 other functions, some becoming long and stiff for flight and steer- 

 ing and some taking on new shapes and colors simply for orna- 

 ment, every change which made the coat of contour-ff'atbers 

 more compact was a distinct gain to the bird. If anv feather 

 with a well-developed aftershaft be examined, it will be seen that 

 the smaller shaft lies exactly underneath the larger und its vanes 

 are closely appressed to those of the main feather, thus practically 

 doubling its thickness and increasing its heat-retiiining power 

 with the least possible loss of compactness. Now, on the other 

 hand, if a feather is examined which has only a very small after- 

 shaft, the latter is not closely appressed to the main shaft and 

 adds almost nothing to its thickness or warmth. Exceptions will, 

 of course, be easily found to these rules, but the fact remains that 

 where the aftershaft is vigorous it gives plain evidence of adding 

 warmth to the plumage, while it is clear that it can have no sec- 

 ondary function of ornament or locomotion. Another reason for 

 believing that the aftershaft is functionally of no importance, ex- 

 cept when an assi.''tance to greater warmth, is found by examining 

 the list of birds which lack it. They are as follows: — 



1. Some Ratitse (Ostriches, Rheas, Apteryx?). 



3. Diomedinse (Albatrosses). 



3. Steganopodes (Gannets. Pelicans, Cormorants, etc ). 



4. Lamellirostres (Ducks, Swans, Geese), except Flamingoes and 

 some Ducks. 



5. Columbse (Pigeons). 



6. Cathartidas (American Vultures). 



7. Striges (Owls). 



8. Pandionidse (Ospreys). 



9. Cuculidse (Cuckoos). 



10. Alcedinidse (Kingfishers). 



Since there are known to science to day about ten thousand 

 species of birds, only one-tenth of which are included in the above 

 list, the absence of an aftershaft may certainly be considered ex- 

 ceptional. If it is functionally of any importance, why should it 

 be wanting in the albatross, though present in the petrel? Or 



wanting in many ducks and present in others? And indeed to 

 assign it any function common to all birds except to those in the 

 above list, while wanting in all of them, will be readily found 

 impossible. That it may be the cause of greater warmth receives 

 a proof of negative value from our knowledge that, while very 

 large in all the other Ratitse, it is totally wanting in those species 

 which inhabit the open plains and deserts of the tropics, where it 

 is not desirable to retain too much heat. Still further evidence 

 appears in the facts that all of the birds which lack an aftershaft 

 (except owls and pigeons) are supplied with a thick coat of down 

 beneath the contour-feathers, and all of the groups except Striges, 

 Lamellirostres, and a few Steganopodes are most largely repre- 

 sented in the tropics and warmer temperate countries. While 

 much of this evidence is very general, some of it purely negative, 

 it seems undoubtedly true that the aftershaft, when not serving 

 as an additional heat-retainer, is wholly functionless. 



The conclusion is now unavoidable that the aftershaft, if func- 

 tionle.ss, must, according to our original proposition, be undergoing 

 a process of gradual atrophy. That such is the case admits of 

 little doubt It must, however, he kept in mind that its possible 

 function as a heat-retainer is admitted, and in cases where this 

 function has been sufficiently exercised, atrophy, if ever begun, 

 has been stopped. Illustrations of this may be found all through 

 the class, but two will be sufficient to show the point. In the 

 Casuaridae (Cassowaries) the aftershaft is of equal size with the 

 main shaft, and its function is undoubtedly the same. It is prac- 

 tically a second feather, and, since compactness of plumage is of 

 no advantage to non-fiying birds, it has continued to exercise its 

 function, and atrophy has never begun. In the Gallinse, one of 

 the oldest and most generalized groups, where compactness of 

 plumage is very desirable, not only because it creates less friction 

 in flight, but also because, being essentially ground birds, they 

 are greatly exposed to cold and damp, the aftershaft is large and 

 thick, but entirely different from the Cassowary's. Increasing 

 compactness of the plumage has greatly modified it, but ati'ophy 

 has not occurred because it still exercises to an important degree 

 its function as a heat-retainer. In the Passeres, on the other 

 hand, the condition of the aftershaft shows evident loss of func- 

 tion and consequent atrophy, being very small and weak. 

 Perhaps in no better way can the degeneration of the aftershaft 

 consequent on its loss of function be proven than by an examina- 

 tion of the feathers of the wing. As is well known, the chief 

 function of the primaries and secondaries is no longer heat- 

 retaining but locomotive, and they entirely lack an aftershaft in 

 all flying birds. But this change of function has undoubtedly 

 been brought about gradually, and on the elbow of the wing are 

 several feathers, very slightly different from the contour- feathers, 

 which grade by almost imperceptible differences into the fully-de- 

 veloped secondaries. If these feathers are examined in any birct 

 with aftershafted plumage, as, for example, the ruffed grouse 

 (Bonasa umbellus), a very evident aftershaft will be found en the 

 smallest ones, but decreasing rapidly in size as the main shafts 

 become flight-feathers, until, on the true secondaries, they are 

 either wholly wanting or represented only by a slight meeting of 

 the vanes. 



It will be noticed that throughout the preceding argument, the 

 assumption has been made that the aftershaft is a degenerated 

 and not a recently evolved part of the feather. That is, that it 

 was originally characteristic of feathers in general and its condi- 

 tion as known to us is worse than formerly, rather than that it is. 

 an acquired character, which never occurred where it is now 

 wanting. The truth of this assumption must now be proven, or 

 the foregoing statements are meaningless. The first reason to be 

 offered for believing it to be a primitive part of the feather is 

 found in the facts of its occurrence and development in the differ- 

 ent orders of birds. In the Casuaridas, which is admitted to he- 

 one of the very oldest families of modern birds, we find a very 

 large aftershaft, indeed, as already said, it is practically one-half 

 of the feather. In the other Ratitae, in which it is wholly want- 

 ing, local causes, such as excessive heat, have destroyed its use- 

 fulness, and its loss is easily explained. Large aftershafts are 

 also found in Opisthocomus, the Gallinae, and the Penquins, ali 

 old and little specialized groups; while, on the other band, in the 



