October, 1903.] 



KNOWLEDGE 



221 



Dr. Wallace brink's forward some new points which are 

 not, however, germane to the question. Several writers in 

 objecting to his statement that if tlie stellar uuiver.se were 

 infinite in extent the entire sky would be a blaze of star- 

 light, made the very sufJicieut answer that the same line of 

 argUTuent if applied to the dark stars would lead to an 

 opposite conclusion. The reply was amply sufficient for 

 its purpose, but Dr. Wallace tries to answer it as if it had 

 been brought forward, not as a mere argiimentum ad hoc, 

 but as an actual the<^ry of the universe, and urges that if 

 the dark stars were so numerous we should frequently 

 observe occupations of the lucid stars. A very little calcu- 

 lation shows that even if the dark bodies were a thousand 

 times more numerous than the bright, the chances are 

 millions to one against any diminution of the light of a lucid 

 star arising from this cause ever having been observed. 



Another point is that though the sun is moving with 

 prodigious speed, yet that the action of gravity would 

 prevent it wandering far from its present position. Why 

 should it ? It has had no such action upon Arcturus, 

 and other "runaway" stars. Then Dr. Wallace raises the 

 question of " star-drift " ; that is to say, of groups of stars 

 moving with a common i^rojjer motion. In what way this 

 lieljjs his argument does not appear. The reference to the 

 five stars of Ursa Major is a particularly unfortunate one, 

 since this group, extending over nearly twenty degrees of 

 arc, is obviousI_v moving as a system in a plane which is 

 nearly at right angles to that of the Milky Way. The 

 latter plane, therefore, is not the only one of high impor- 

 tance within the limits of the visible sidereal system. 



It is disappointing that Dr. Wallace takes no notice of an 

 exceedingly suggestive point raised by Prof. H. H. Turner. 

 We speak roughly of the Galaxy as forming a ring. The 

 researches during the last half-century of Heis, Boeddicker, 

 Backhouse, Stratouoff, Easton, and others, have shown us 

 that it is about as unlike a simple annulus as any object 

 could possibly be. Tt is an object of the greatest com- 

 plexity, formed of long irregular branching streams, inter- 

 lacing and crossing one another, and some of them reaching 

 out far towards its poles, of close agglomerations side by 

 side with broad lacunae. But, most striking of all, there 

 are two portions — if portions they be and not separate 

 and external galaxies — which stand out by themselves and 

 away from the main body — the two Magellanic Clouds. 

 If they are truly part of the Galaxy, then we are no longer 

 in a position to assert that we are in its medial plane or 

 near the centre of that plane. If they are external galaxies, 

 then our Galaxy is not the sole one known to us ; the 

 visible universe evidently extends much beyond it. 



But if we di I, hold a " nearly central " position, Dr. 

 Wallace's question, " What advantages have we derived 

 from it ? " would still be wholly unpractical, and to 

 complain there are " hardly any suggestions of enlighten- 

 ment in astronomical literature, but, rather, what seem to 

 me now to be unnecessary difficulties thrown in the way 

 of the enquirer," is much as if a man took a candle in 

 order to read the time of night from a sundial, and com- 

 plained that the literature on dialling gave niJ guidance 

 how to proceed in such circumstances. 



Practically, Dr. Wallace's jjosition in his second paper 

 amounts to this. He has withdrawn as untenable the 

 propositions upon which his original thesis was based ; 

 but in effect he claims the right to maintain his former 

 conclusions until his critics liave demonstrated propositions, 

 the opposite of all tliosc lie has advanced. 



To sum up. Wc have no sufficient evidence to show 

 whether the stellar universe lias an iudefinito extension or 

 not ; or, if it be bounded, whether we have yet penetrated 

 to the boundary. Supposing such a boundary, we have 

 not the slightest reason to suspect, any stiir that we can see 



of being in an unstable condition owing to its nearness to 

 it. We do not know whether the Galaxy includes in its 

 structure the whole of the olijeets which we see, or whether 

 any considerable number lie beyond it and are of a different 

 formation. We do know, and it has long been known, 

 that our sun is near the medial plane of the Galaxy, and 

 probably not more than twice as far from one side of it as 

 from the other. But we do not know that it is nearer the 

 centre of the Galaxy than hundreds of other stars, nor 

 have we the slightest reason to suppose that the svstems 

 attendant upon them are less fitted to be the home of 

 intelligent life than our own. 



THE CLAWS ON THE WINGS OF BIRDS: 



A STUDY IN EVOLUTION. 

 By W. P. Pycraft, a.l.s., f.z.s., etc. 

 It may not be generally known that, hidden away among 

 the feathers of a bird's wing there are to be found 

 frequently two tiny claws, one on the thumb, the other on 

 what corresponds to the first finger of the human hand. 

 According to the text books these are to be regarded as 

 relics of a reptilian ancestry ; mere survivals of an order 

 of things now obsolete. Just as the gill-sUts in the 

 mammalian embryo point to a fish-like aquatic stage of 

 development long since suppressed ; or as the vestiges of 

 hauneh and thigh bones buried deep in the muscles of the 

 belly of the whale point to a time when functional hind 

 limbs were present. 



The very best of reasons may be urged for accepting 

 this hypothesis, inasmuch as in the most ancient, and at 

 the same time most reptilian, of all known birds — the 

 lizard-tailed Archmopferyx—a,s well as in the Giant 

 Ostriches, which are admittedly primitive, these claws are 

 very large, whilst in the most modern types they are 

 always small, or wanting. Again, like all vestigial organs, 

 the method of their going is slow and more or less orderly. 

 They beat, as it were, a dignified retreat, taking their 

 dismissal with reluctance. Thus, onl3- in the primaeval 

 Arch(eopteryx do we find the full complement of claws, one 

 to each of the three digits— all that remains of the h;ind 

 in the bird's wing. The first to disappear was the claw on 

 the third digit. This has now completely vanished from 

 the adult life of living birds, and only occasionally appears 

 in the embryo of the Old-World ostrich. Growing gradu- 

 ally smaller and smaller, the remaining two frequently 

 appear only during embryonic life, and may cease, like the 

 vanished number three, to be reproduced even here. The 

 gradual stages in this work of demolition, so invariably 

 associated with vestigial structures, seemed, we may repeat, 

 the best of evidence for regarding these claws as mere 

 survivals of a reptilian origin, and of no other significance 

 or purpose within avian times. 



The axe must, however, occasionally be laid to the roots 

 of the most promising hypothesis, and sometimes the 

 warrant comes from most unexpected quarters. Even 

 from " babes and sucklings," as in the present case — to 

 wit, the young of the aberrant South Americ;ui game-bird, 

 known as the Hoatzin. Clawless in the adult, the wing 

 in the nestling, on the contrary, is found to be provided 

 with claws of an unusually large size. This fact, nothing 

 being known of tlie life-history of this bird, would have been 

 interpreted as an instance of organs which had for some 

 unknown reason resisted the decadence which had over- 

 taken the same structures in other species, and here the 

 matter would have rested. Fortunately, however, we are 

 in possession of a very complete account of the habitat 

 and breeding habits of the Hoatzin, due lai-gely, indeed 

 principally, to the observations of Mr. Quelch. He has 



