34 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



16. Some Physiologists have asserted, that the nature of the respiratory 

 process affords a ground of distinction between Animals and Plants ; oxygen 

 being absorbed, and carbonic acid evolved, by the former, and a converse 

 change being effected in the surrounding air by the latter. It is not correct, 

 however, to designate this converse change as a consequence of the respiratory 

 process ; for in Plants, as in Animals, there is a continual absorption of oxy- 

 gen and evolution of carbonic acid, which constitute the function of respira- 

 tion ; but the effects of this change are masked (as it were) by those of the 

 process of fixation of carbon from the atmosphere, which only takes place 

 under the influence of sun-light, and which is much more analogous to the 

 digestion of Animals. The most valid distinction, in doubtful cases, seems 

 likely to be founded on the chemical constitution of the tissues themselves. 

 In the Plant, the whole of the organized structure, when freed from the pro- 

 ducts of secretion which are deposited in it, (many of these containing the 

 same proportion of nitrogen as exists in animal flesh, 454,) is found to have 

 the same composition with starch ; being formed of oxygen, hydrogen, and 

 carbon only. In the Animal, on the other hand, the organized tissues all con- 

 tain azote as part of their proper substance ; non-azotized compounds, such as 

 fatty matter, being merely deposited in these, as products of secretion. Hence 

 if the chemical composition of the organized tissues themselves can be cor- 

 rectly determined, the Vegetable or Animal nature of a doubtful body may be 

 ascertained. By this test the long-disputed question of the nature of the true 

 Corallines has been set at rest ; their tissue, when freed from the lime depo- 

 sited in it, being found to have the composition of that of Plants.* 



General Subdivisions of the Jlnimal Kingdom. 



17. The Animal kingdom was formerly divided into two primary groups, 

 the Vertebrated and the Invertebrated; the former comprising those which are 

 distinguished by the possession of a jointed spinal column, consisting of a 

 number of internal bones termed vertebrae ; and the latter including all those 

 animals, which are destitute of this support. It was pointed out by Cuvier, 

 however, that, among the Invertebrata, there are three types of organization, 

 as distinct from each other, as any of them are from the Vertebrata ; and he 

 accordingly distributed the whole under four primary divisions or sub-king- 

 doms : of these, the VERTEBRATA rank highest ; next, the ARTIUULATA and the 

 MOLLUSCA, which are both inferior in degree of organization to the Vertebrata, 

 but are superior to the lowest group, the RADIATA, which contains those ani- 

 mals that border most closely, both in external aspect, and in general character, 

 upon the Vegetable kingdom. The members of these groups are readily sepa- 

 rated from each other by the structure of their skeletons, or organs of support 

 and protection ; as well as by many other characters. In the Vertebrata, the 

 skeleton consists of a number of internal jointed bones, which are clothed by 

 the muscles that are attached to them and move them ; these bones are tra- 

 versed by blood-vessels and absorbents, and are to be regarded as in all re- 

 spects analogous to the other living tissues of the body. In the Articulata, the 

 soft parts are supported by a hard external envelope, which is of corresponding 

 form on the two sides of the median line, and is divided into several pieces, 

 jointed or articulated together by a membrane, in Such a manner as still to 

 allow of free motion ; and the muscles, which are numerous and complex, are 

 attached to the interior of these. In the Mollusca, the whole body is quite 

 soft ; and many species exist, in which it has no external protection : in . a 

 large proportion of the group, however, the surface has the power of exuding 



* See London Physiological Journal, vol. i., p. 29, and Comptes Rendus, July 3, 1843. 



