128 Mr, Edmonston on the Origin of the Power of suspending 



siologists luminously demonstrate ? To all these suppositions 

 the same general objection applies, — that they arenotonly en- 

 tirely destitute of proof, and inadequate to the solution of the 

 difficulty, but multiply others equally unaccountable. While 

 therefoi'e the most minute and laborious anatomy has failed 

 to unravel any peculiarity of structure adequate to account for 

 this phEenomenon, the doubt naturally arises, whether in every 

 case of difference or modification of function we are entitled 

 to presume difference of structure; whether the faculty in 

 question may not be one of those modifications of the vital 

 principle independent of tangible peculiarity of organization ? 

 And under this impression, the hypothesis which appears to 

 me to be the simplest, and to be supported by analogy, is, that 

 the conditions of the nervous system of aquatic animals are 

 such, that venous blood requires a much longer period to cir- 

 culate through their brain than in that of land-animals, to 

 produce the same deleterious effects. Why should it be mat- 

 ter of surprise, that the brain of aquatic animals should have 

 peculiar relations to certain fluids circulating through it ; or 

 that their nervous system should be less or more susceptible 

 of sedative or stimulating influence ? Different animals, and 

 even different organs in the same animals, are very differently 

 affected by similar causes ; and in such cases we do not always 

 necessarily presume difference of structure. Is it even won- 

 derful that the excitability of aquatic animals should be such 

 as to render them unable permanently to bear the same highly 

 oxygenized blood as land-animals ; much in the same way as 

 we suffer, and should at length die, by breathing air in which 

 there is more than its usual proportion of oxygen ? Different 

 species of terrestrial animals require, cceteris paribtis, unequal 

 quantities of oxygen; and even different individuals of the 

 same species, both in health and disease, are dissimilar in this 

 respect. 



Why may not the natural and healthy state of the blood in 

 aquatic animals have a tendency to the venous, as that of ter- 

 restrial animals has to the arterial standard ? And this indeed 

 I believe to be the fact; for in all amphibious mammalia and 

 birds which I have examined, the blood has much more of 

 the venous than arterial appearance. If this fact be established 

 by a sufficiently ample induction, it would seem decisive of 

 the question ; and it would furnish a satisfactory reason why 

 aquatic animals decline and at length die, when placed in si- 

 tuations where they are long deprived of the power and the 

 necessity of diving. 



I have also observed, that if seals, after remaining on the 

 rocks for an hour or two, are killed before returning to the 



water, 



