103 Mr Carpenter on Unity of Function 



on which tlie brancheas of aquatic animals are constructed, 

 whatever may be the modifications of their form and situation. 

 In air-breathing animals, on the other hand, the prolongation of 

 the surfaces takes place internally, so that the air comes to 

 meet the blood, instead of the blood being sent to meet the 

 air. Figs. 1 and £ therefore will equally well serve as repre- 

 sentations of these two principal types of the development of 

 the respiratory system. We find, however, many interesting- 

 intermediate forms, such as the pulmonary branchiae of the 

 Arachnida ; and in tracing the development of the air-bladder 

 of fish into the lung of the reptile, and at the same time the 

 progressive disappearance of the gills, we have a beautiful ex- 

 ample of the gradual change which (where the links are all 

 within our reach) may everywhere be observed throughout na- 

 ture. I think that the structure of the respiratory organs af- 

 fords a beautiful illustration of the argument which might be 

 raised on a priori considerations in favour of the doctrine of 

 " fundamental unity of structure."* The function of respira- 

 tion is a very simple one, and it is essentially the same not only 

 throughout the animal kingdom, but in vegetables also, as I 

 shall presently shew. It might be regarded then, as a neces- 

 sary result of the law, which everywhere prevails throughout 

 creation, of the attainment of every end by the best adapted 

 means, that the essential structure of the organs should be the 

 same whei'e the function is the same, but that the disposition 

 of these parts should vary with the circumstances in which that 

 function is to be performed. I need not point out the evident 

 correspondence of this conclusion with existing facts. 



The experiments of the late Professor Bennett and Dr Dau- 

 beny, on the gaseous changes produced by vegetables, warrant 

 (I think) the conclusion, that the disengagement of carbon, 

 which by union ^y\\^\ the oxygen of the atmosphere forms car- 

 bonic acid, is constantly going on, and is essential to life equal- 

 ly with the respiration of animals ; while the fixation of carbon, 

 which only takes p'ace during the stimulus of sunlight, is rather 

 analogous to the digestion of animals. The latter process in a 

 healthy plant far more than counterbalances the other ; and 



* This term I derive from Dr Barry. 



