THE CONDITIONS OF LIFE IN ANIMATED BEINGS. 



427 



body and limbs ; it arises also from the extension 

 of the respiratory apparatus, and from the force 

 of circulation of the blood. If a pheasant were 

 swept along with the rush of a falcon, or a spar- 

 row with a swallow's swiftness, the unhappy 

 pheasant and wretched swallow would be blown 

 immediately, and would soon fall helpless. With 

 birds, the size of their air-receptacles is always 

 in perfect relations with the degree of activity, 

 the energy of movement, and power of flight. In 

 this respect, comparative study, which has not 

 yet been made thoroughly enough, would furnish 

 remarks of great interest. With an increased or 

 diminished activity of respiration, the circulation 

 of the blood, too, shows corresponding variations, 

 within limits fixed by the structure or the ar- 

 rangement of the organs. In the great fliers the 

 heart is larger in proportion to the size of the 

 body than in sedentary species. The left ven- 

 tricle, which propels the vital fluid into the whole 

 arterial system, has walls of considerable thick- 

 ness, supported also by strong muscular columns, 

 in birds of powerful flight, where its contractions 

 need to be made most energetically. It is curi- 

 ous to follow in examination the whole series 

 of slight variations in ducks, cranes, flamingos, 

 gulls, and birds of prey, in which last the highest 

 degree of perfection is realized. In species that 

 have a feebly-sustained flight, as gallinaceous 

 birds, parrots, or sparrows, the same walls and 

 muscular pillars of the heart present in compari- 

 son only a weak resistance. In the same way 

 the capacity of the right ventricle, into which the 

 blood flows from the veins, is modified : of mod- 

 erate size in those species of quiet habits, it is 

 large in species more energetic in movement and 

 capable of performing long journeys. 



In old times simple persons had the notion 

 that, by merely fastening wings to their shoulders, 

 they could succeed in rising into the air. If the 

 idea was really carried so far as the beginning of 

 execution, the attempt must have at once satis- 

 fied the most enterprising of the emptiness of the 

 project. Man has not the strength to manage 

 great wings ; and, if he had the strength, the pro- 

 portion and weight of his body would remain in- 

 vincible obstacles. The bird, completely covered 

 with feathers, admirably shaped for its usual 

 mode of moving, has muscles of enormous 

 strength to supply motion to its fore-limbs ; and 

 it presents but little weight, for its body contains 

 great pockets always full of air, and its bones, for 

 the most part, are hollow. In our days, the idea 

 of navigating the air comes up again constantly ; 

 there are inventors who usually give themselves 



very little concern about the views of science, 

 and who, nevertheless, are well satisfied of the 

 possibility of success. Does not the model seem 

 to exist in Nature ? But it is that model precise- 

 ly that inspires the naturalist with the fear that 

 the pursuit is chimerical. The size of an eagle 

 or a condor is not very considerable, and the 

 bird which attains still greater dimensions, yet 

 without losing any of the essential characteristics 

 of the type to which it belongs, is incapable of 

 flight. The ostrich and the cassowary stay on 

 the ground ; the gigantic dinornis, which still in- 

 habited New Zealand a few years ago, did not 

 fly ; the epyornis of Madagascar, whose enor- 

 mous eggs have been the theme of astonishment, 

 and almost of admiration, was not more favored 

 than the other birds mentioned. Thus the ob- 

 servation of what exists in Nature leads us to 

 think that locomotion through air is incompati- 

 ble with great size. 



We do not intend to select instances from all 

 the classes of the animal kingdom of coincidences 

 between specialties of organization and aptitudes ; 

 but there is one which invites citation, because it 

 relates to animals habitually in every one's sight. 

 A carp lives comfortably in a narrow reservoir 

 with muddy water seldom renewed ; a trout thrown 

 into the same water dies asphyxiated in a few min- 

 utes ; for a trout requires running water always 

 well aerated. The carp consumes little oxygen; 

 its respiration is weak. A trout's respiration is 

 far more active. The difference in function is ex- 

 plained by certain arrangements in the branchiae 

 and the apparatus of circulation, and then we 

 understand the necessity for the trout of an en- 

 tirely different abode from that of the carp. 



Among the remarkable peculiarities of the 

 life of beings, there are few more instructive than 

 the exceptions presented in a large number of 

 natural groups. Thus, if the representatives of 

 one class are usually land-animals, some, neverthe- 

 less, dwell in the water; if a class is composed 

 of species essentially aquatic, several species of 

 that zoological division have the power of escap- 

 ing from their element ; such a difference in the 

 conditions of existence does not ordinarily require 

 a profound modification of the organism. We 

 are here struck by the simplicity of the means 

 used by Nature to obtain an important result. 

 Among fishes and Crustacea, animals so admira- 

 bly formed for their usual mode of life, there are 

 species which, voluntarily or by accident, pass a 

 part of their existence out of water. In aquatic 

 animals death occurs as soon as the organs of res- 

 piration, ceasing to be moistened, begin to dry up. 



