SCIENCE. 



553 



In the incessant battle for existence among animals the 

 possession of powers of flight are, in certain directions, 

 so specially advantageous, that any accident of confor- 

 mation, aiding animals in any degree to support them- 

 selves in the air, seems very likely to be preserved and 

 to be added to. At present, however, when the field of 

 air is so fully occupied, and there are so many vigorous 

 carnivorous birds constantly seeking for poorly defended 

 food animals, the tendency to develop powers of flight 

 is checked. The aerial motion of the leaping animals 

 mentioned is not such as to greatly expose them to dan- 

 ger from this source. But were their powers of flight 

 increased, so that they could support themselves longer 

 in the air, and move out from the immediate shelter of 

 trees, they would be exposed to attacks from carnivorous 

 birds, and such an imperfect flight must prove a special 

 disadvantage. Their weak flight would expose them to a 

 danger from which they could escape neither by rapidity of 

 motion, nor by a return to the shelter of the forest. Thus 

 the existence of these strong carnivorous birds operates 

 as a decided check on the development of any other 

 form of flight, and it is very unlikely that the leaping 

 creatures mentioned will ever develop a more efficient 

 flight. 



There is another illustration of the same principle. 

 The true flying mammals, the bats, perhaps developed 

 their powers of flight at the same geological age as 

 birds did. Thus there was no special hindrance to their 

 evolution. It might seem curious to some, however, 

 that they have failed to gain the same extensive habitat 

 as birds ; that they are exclusively crepuscular j or 

 nocturnal in their habits, while the birds are almost ex- 

 clusively diurnal. And yet this difference must have 

 arisen in consequence of a long-continued conflict be- 

 tween bats and birds, in which birds conquered. If we 

 consider that the flying organs of birds are more efficient 

 than those of bats, the whole question is answered. The 

 birds drove the bats back into a field which they did not 

 care to occupy. The fight was for the possession of the 

 air during the day. The birds conquered. The bats 

 were forced to content themselves with nocturnal flight 

 and the imperfect food-supplies which the night yields, 

 while the birds proudly held the dominion of day. It 

 was a case somewhat similar to that which we have al- 

 ready considered, of the supremacy of mammals over 

 reptiles ; and the disappearance of the flying Pterosaurs 

 of the far past may possibly have been due, both to the 

 superiority of flying powers and to the more efficient 

 vasculrr system of birds over flying reptiles. 



But there was a period in which the haired animal had 

 not yet succeeded the scaled animal, in which hairs had 

 not been specialized into feathers, and in which the broad 

 fields of air were free to the first occupant, and contained 

 no active carnivorae ready to check the development of 

 flight in its incipient stages. Were such the case now, 

 our Flying Squirrels, etc., might gain more efficient pow- 

 ers in this respect, and evolve true flight. We may safely 

 come to this conclusion from the fact that certain lizards 

 gained powers of flight in the early days of bird evolu- 

 lution, and apparently by a direct continuation of the 

 process which we perceive in the Flying Squirt els. The 

 expanded skin between the legs was merely a thin, naked 

 membrane in these lizards. It apparently spread wider 

 and wider, until it extended to the extremities of the toes. 

 But this condition would only produce a more efficient 

 parachute motion. For flight to arise a movement of 

 the fore limbs must be employed, and this may have 

 originated in the effort to keep the body horizontal and 

 prevent its anterior extremity from sinking. Such a 

 movement, with the possession of such a membrane, 

 would yield an imperfect flight. But for the flight to be- 

 come perfect, in such an animal, the membrane must be- 

 come still more extended, and the movement of the fore 

 limbs more efficient. This extension was gained by the 

 gradual outgrowth of the fourth finger, to whose tip the 



membrane had already extended. By the preservation of 

 favorable variations in this direction, it is very likely that 

 the immensely extended flying finger of the Pterodactyle, 

 the bird-like conformation of its skeleton, and its proba- 

 bly more efficient lung action, were gradually gained ; 

 and that it thus developed from an original tree-leaping 

 into a flying reptile. 



The advantages of an aerial residence, at that early 

 period when the atmosphere had not yet been preempted 

 by vertebrate inhabitants, were so many and great, that 

 the selective principle must have proved of particular 

 efficiency in this direction. Even a long leap from a 

 solid support infested by many enemies, through an aerial 

 field devoid of enemies, was highly advantageous, and 

 the development of flying powers may have been pre- 

 ceded by the appearance of many animals possessed of a 

 parachute motion. Such animals would leave no geo- 

 logical record, as their motion did not necessarily modify 

 the osseous framework of the body. There are compara- 

 tively few of them now, since the advantage to be gained 

 by such powers is greatly reduced. But it is very obvious 

 that any extension of this power, assimilating it to true 

 flight, must have proved far more advantageous. An 

 animal capable of supporting itself in the air, at this 

 early period, was exceptionally free from danger, while its 

 chances to obtain food were greatly increased. The 

 better flyers were, of course, the more secure, and the 

 original partial flight must have rapidly developed into 

 the most perfect flight of which such a membranous wing 

 extension was capable. Not until, these flying reptiles 

 began to prey on each other was there any check to their 

 evolution and diversity of formation. The remains of 

 some twenty different species have already been discov- 

 ered, and this is indicative of hundreds, perhaps thou- 

 sands, of diverse species of these strangely flying Ptero- 

 saurs. 



A somewhat similar instance of early and perhaps 

 general possession of the atmospheric field is afforded in 

 the case of insects. Some peculiarity of ormation, per- 

 haps the remnant of an original dorsal gill, gave them a 

 structural organ capable of partly aiding them in leaping 

 and of being modified into a membranous wing. But as 

 in vertebrates so in insects, the field of air became in 

 time so fully occupied, and the claimants for its food 

 supply so numerous, that the less vigorous flyers or those 

 adapted to specially terrestrial food, descended to the 

 earth again, and in time lost the power, with the loss of 

 the desire or need, of flight. 



Some such fate seems to have overtaken the Pterosaurs. 

 For the possession of the field of air was sharply con- 

 tested by two other classes of animals, both of which 

 had already stepped beyond the reptilian rank, and be- 

 come hair-covered and warm-blooded, and one had 

 gained the still more advanced condition of the mam- 

 malian organization. The contest between birds, bats, 

 and Pterosaurs, was not alone a battle of flying car- 

 nivoraj, but a struggle for common supplies of food, in 

 which the most vigorous flyer was most likely to win. 

 As a probable consequence of this contest the birds have 

 gained the diurnal possession of the air, the bats have 

 become adapted to nocturnal food supplies, and the 

 Pterosaurs have disappeared. They were probably van- 

 quished in the fight, and gradually resumed terrestrial 

 habits, or died out. 



So far as we are aware the evolution of flying mam- 

 mals was much later than that of birds. But the geo- 

 logical record is so incomplete that bats may have existed 

 at a much earlier date than their discovered remains seem 

 to show. They are found in Eocene deposits, and the 

 bird remains yet found in earlier deposits are very few 

 in number of instances or species. In considering the 

 development of bat flight w<; are in the same line with 

 those already examined. It is a development of the skin 

 into a flying membrane. But in this case, in addition to 

 the extension of this membrane between the fore and 



