OCTOBEB, 1903.] 



KNOWLEDGE. 



223 



organ of flight. The explanation of this accelerated 

 development of the quills is not far to seek — it is the 

 result of adaptation to the changed environment. The 

 descent from the trees to the ground was a descent from 

 comparative security from enemies into a world where 

 enemies were numerous. Precocious flight was the method 

 of escape adopted, though, as we shall see presently, not 

 the only method. 



But how comes it, some may ask, that the arboreal 

 nursery was forsaken, if it afforded such security from 

 enemies ? And, further, how is it that the young of all 

 birds hatched in trees at the present time — save only the 

 Hoatzin — are so singularly helpless at birth ? 



As touching the migration from the forest to more open 

 ground, we may surmise that this probably took place as 

 a result of overcrowding. The old habitat left behind, the 

 young in response to the new environment underwent 

 modification, now in one direction, now in another, to 

 bring them into harmony with their particular require- 

 ments. 



Among the forms known as Game-birds, and the 

 Tinamous, the most striking of these changes is that 

 affecting the wings, the nature of which we have just 

 sketched in the wing of the common fowl. But we 

 would draw sjjecial attention to one further point con- 

 cerning the accelerated development of the quills. This 

 forcing, it is instructive to note, affected only those quills 

 originally concerned in the precocious flight ; those at the 

 tip of the wing, whose development was retarded so as to 

 leave the claw free for climbing purposes, still remaining 

 unaffected. This is as we might expect, for just as these 

 inner quills were sufficient during the arboreal phase, 

 they remained and still remain, equally so for all the 

 demands of the terrestrial life. 



The fact that similar traces of an arboreal life are not to 

 be found in the precocious young of birds other than 

 Game-birds, is a curious and extremely interesting point, 

 and not only reveals a change in the tactics adopted for 

 the escape of enemies, but draws attention to another and 

 what we may regard as a second string to the bow, 

 practised by the Game-bLi-ds themselves. It must be 

 remembered then that these latter are reared in compara- 

 tively large families, and that they in consequence afford 

 a conspicuous and tempting prey to prowling carnivora. 

 Accordingly, as soon as danger is realized by the parent, 

 the alarm is given and the young scatter in all directions. 

 Halting at last, they then fall back upon this " second 

 string" — protective coloration. That is to say, they have, 

 in addition to the remarkably accelerated flight, also 

 acquired a peculiar type of plumage which enables them 

 to assimilate with the surrounding objects. Now it would 

 seem that this precocious power of flight has not proved 

 a really satisfactory method of escape, inasmuch as in 

 fleeing from immediate danger, the young either strayed 

 too far to render recall possible, or they fled into new 

 danger. Consequently, the young in other groups have 

 come to rely either on protective coloration alone, or at 

 most run but a few yards and then s(|uat down. Or, as 

 in the case of one of the Coursers, the parents cover the 

 young with earth. On this account, then, other groups 

 "have discarded the doubtful refuge afforded l)y precocious 

 flight, and with it the evidence of these earlier arboreal 

 habits. The young of aijualic birds obviously do not need 

 to seek safety in flight. CoiuTaliiicnl amid reed-beds or 

 other vegetation affords ain|>le proteclion. Among all 

 these non-flying young we find the development of the 

 quills luis been retarded rather than accelerated, so that 

 they appear together with the rest of the body plumage. 

 Among the ducks, indeed, the quills do not appear till 

 extremely late, so that the body has attained almost 



its full size before the wing begins to attain its adult 

 form. 



The wing of the nestling Rhea — the South American 

 Ostrich — still retains traces of evidence of a developmental 

 history, precisely similar to that of the forms which we 

 have been discussing. Whether such traces will be found 

 in the other flightless meml)ers of the Ostrich tribe, 

 remains to be seen. In the Tinamous, the only Ostrich- 

 like birds which have retained the power of flight, the 

 development of the wing is precisely similar to that which 

 obtains among the Game-birds. 



This is a point of some considerable importance, since it 

 shows that, as we have reason to believe on other grounds, 

 the giant members of the Ostrich tribe have attained their 

 present conspicuous bulk comparatively recently, that is 

 to say, since they became flightless. That they are primi- 

 tive types there can be no possibility of doubt, but, like 

 other primitive types, their great size is the last develop- 

 mental phase in their life-history, and precedes extinction. 

 But of this more anon. 



It is time that we turned to the opposite side of this 

 picture — to the consideration of those types of nestlings 

 which are ushered into the world blind, naked, and 

 helpless. According to the terms of our argument, birds 

 were originally a strictly arboreal group, and their young, 

 like those of" reptiles, were extremely active from the 

 moment they left the shell. 



Without doubt, such activity in an abnormal nursery 

 must have been attended by considerable infant mortality 

 through the young falling to the groimd. Many, probably, 

 would fall through weakness ; the habit of dispersing 

 themselves among the branches of the trees in which the 

 nest was placed resulting in a loss of regular food supply, 

 owing to the ditKculty of being on the spot when the 

 parents returned with food. 



Now two courses were open whereby this infant mortality 

 could be reduced. Either the eggs could be deposited on 

 the ground, or the activity of the young could be curtailed. 

 The game-birds, ducks and geese, rails, cranes and plovers, 

 may serve for examples of those species which have de- 

 scended from the trees to the ground for nesting purposes. 

 Although, as a consequence, such young have undergone 

 cousideVable changes in adaptation to their new environ- 

 ment, these changes are not so striking as those T,vhich 

 have taken i)lace among the young of the species which, 

 retaining the ancient practice of nesting in the tree tops, 

 have adopted the alternative of curtailing the activity of 

 their offspring. This curtailment was accomplished by 

 reducing the amount of food-yolk enclosed within the egg. 

 As a consequence of this reduction the embryonic period of 

 development has become relatively shortened, and the 

 young in consequence emerge from the shell in the helpless 

 condition to which we have referred already. 



The number of species which have adopted this expi'dient 

 outnumber those which have not, and this speaks volumes 

 for its success. As examples we may cite the vast army 

 of song birds, the parrots, cuckoos, birds of prey, 

 cormorants and tiieir allies, and the stork tribe. 



Thoui,'h the voung of these birds are all born in an 

 extremoTy helidess state, they differ, it must be noticed, in 

 one very" interesting particular— the majority, when they 

 leave the shell, are perfectly naked, whilst others are 

 enveh>[)ed in a coat of down, and in some the down develops 

 soon after halcliing. This variability in the matter of 

 plumage furnishes indirect testimony in support of the 

 theory which we set out to maintain. 



The amount of food-yolk once reduced, return to the 

 older fashion of active young became impossible; and 

 this explains wliv helpless young are still born to those 

 parents which have adopt'ed the practice of depositing 



