422 " J. BARBELL INFLUENCE OF CLIMATES ON VERTEBRATES 



less than half way ; in the development of lungs it is much more than 

 half way; in the development of its heart it is at least half way. The 

 gills could not be lost until the whole circulatory system of the selachian 

 had been largely remodeled. The lung preceded this remodeling, and the 

 changes in the heart were made necessary by the increasing disuse of 

 gills. The lung-fish, then, does not show the initial beginnings of air- 

 breathing, but rather that lowest state which could survive from the 

 remote past to the present. 



If a broad control by natural selection were not postulated as the gov- 

 erning cause, it would be necessary to assume that orthogenetic mutations 

 went forward, first, in the development of lungs ; second, in the develop- 

 ment of a new type of heart, and, third, in the atrophy of gills ; and that 

 the first two changes, at least, although in different organs, were blindly 

 working toward the same end, one not at that time established or even 

 apparent in nature. This is too great a demand on fortuitous coincidence 

 and not in accord with the law of probabilities. The pressure of natural 

 selection, operating to a degree at which it threatened the very extinction 

 of the entire race, is demanded to explain the guidance of development 

 and transformation of unrelated organs into mutual support and com- 

 bined efficiency in a new sphere of life. 



PROBABLE ORIGIN OF THE AIR-BLADDER AS AN INTESTINAL DIVERTICULUM 



If now we attempt to reconstruct the stages which led to the develop- 

 ment of lungs, a preliminary survey must be made of the devices for 

 using air which are employed by modern fishes under similar environ- 

 mental conditions, but fishes whose air-bladder had been modified previ- 

 ously to other uses and who were therefore required to evolve new struc- 

 tures to meet this respiratory need. The data regarding accessory organs 

 of respiration as found among teleosts are brought together by T. W. 

 Bridge, 30 and the following are his statements : 



"Accessory organs of respiration. — In certain Fishes of peculiar habits, or 

 living under special external conditions, accessory respiratory organs are de- 

 veloped. 



"Although in this particular instance no special organs are formed, mention 

 may first be made of the singular method of intestinal respiration in vogue in 

 some Teleosts. In one of the Loaches (Misgurnus fossilis), air is swallowed 

 and passed along the alimentary canal until it is finally voided at the anus. 

 The mucous membrane of the intestine is extremely vascular, and hence the 

 blood comes into sufficiently intimate relations with the swallowed air to admit 

 of it exchanging carbon dioxide for oxygen. Intestinal respiration also occurs 

 in species of the South American fresh-water genera of Siluridre and Lori- 

 cariidae, GaUichthys, Doras, Loricaria, and Plecostomus ; and in some cases 



30 Fishes, 1904, pp. 292-295. The Cambridge Natural Science Series. 



