382 Comimratwe Physiology. 



real analogy could exist. The essential character of the func- 

 tion, however, is to bring the circulating fluid (blood or sap) 

 into due relation with the atmosphere ; and all that is needed is 

 a membrane which shall be in contact with the air on one side, 

 and the circulating fluid on the other. In all the forms of 

 respiratory apparatus there is the same essential character, and 

 their modifications are only to adapt them to the conditions of 

 the structure at large. There is, functionally considered, a 

 unity of comi^osition, although not really analogous in structural 

 character. In the vegetable kingdom, organs which correspond 

 in structure, connexions, and developement, are observed to 

 assume the most varied forms, and perform the most different 

 functions. 



" It has been maintained by some physiologists, that the 

 same elementary parts exist in all animals, and the only differ- 

 ence between the various classes is in the respective develope- 

 ment of these parts. This is, however, true only in a restricted 

 sense. In the Vertebrata, the skeleton of the fish may be 

 shown to be composed of the same parts as that of a bird or qua- 

 druped, though the form of individual bones may be totally dis- 

 similar ; the lungs of the air-breathing Vertebrata exist in a 

 rudimentary condition in fishes, some of the higher classes 

 having the rudiments of a bronchial apparatus. Among the 

 Articulata the same correspondence may be traced ; but the 

 classes of this division will not admit of being compared with 

 those of Vertebrata. There are many plants which bear stamens 

 only in one set of flowers, and pistils in another ; and these 

 may be caused to produce flowers entirely perfect, by supplying 

 nourishment enough to develope the rudimentary organs. When 

 any new function, or great modification of function, is to be 

 performed, no entirely new structure is evolved for that pur- 

 pose, the end being attained by a modification in some structure 

 already present. In all the great divisions of organic beings, 

 there is a fundamental correspondence amongst the different 

 organs. Nature appears to have kept in view a certain definite 

 type or standard, to which she has a decided tendency to con- 

 form, and departs from the original plan only to accommodate her- 

 self to certain specific and ulterior objects peculiar to particular 

 races of created beings. This unity of composition, however, is 

 sometimes interfered with, by the tendency of one division to 

 approach to another, producing organs characteristic of an ap- 

 proximate division. The functional character of the organs 

 furnishes a more general analogy than any we can trace from 

 structure alone. The simplest plant differs from the most com- 

 plex, principally, in that the whole surface participates in all the 

 operations of absorption, exhalation, and respiration, which con- 

 nect it with the external world ; while, in the more complex 



