II. The blood of Reptiles is superior to that of Fishes both as regards the nature of the globules and their relative proportion, their size being smaller, and their numbers greater, than among Fishes.
If the whole of the blood in the Reptile is not transmitted through the organ of respira tion, whilst in the Fish it is, a larger quantity of this fluid is brought into contact with the air in the same space of time in consequence of the greater extent of surface of the organ in the Reptile, and then the Reptile has the farther immense advantage of a pulmonary or aerial respiration, whilst that of the Fish is branchial or aquatic. To conclude, the nervous system of Reptiles is much more developed in the cerebro-spinal axis, and especially in the ence phalic extremity, than in Fishes.
From this comparison it follows that the organic and functional conditions, judging of these in conformity with the principles which we have taken as our guide, are much more favourable to the development of heat in Rep tiles than in Fishes. This theoretical deduction is fully confirmed by direct observation, as we have seen above, and this verification becomes a new confirmation of the accuracy of the prin ciple.
We continue to pursue this parallel by a summary comparison of the organization in its relations with the production of heat in the cold-blooded Vertebrata and the Invertebrata generally. A glance suffices to shew the vast inferiority of the Invertebrata in this as in every other respect. In the first place their blood is so little of the same nature as that which has been recognized most favourable to the produc tion of heat, that it wants the characters whe ther of arterial or of venous blood. The hlood of the Invertebrata, with the exception of a very small group (the worms with red blood), is colourless. In the structure and number of its globules it is also greatly inferior. The globules, indeed, may be smaller, but then they are of a much more simple structure, and con sequently lower in the scale, in other words more imperfect. In the relation to the fluid part of the blood too, they are in much smaller proportion than among the Vertebrata. An
analogous character is manifest in the tissues generally, the proportion of water in them being incomparably larger in the Inverte brate than in the Vertebrate series of animals. Filially, there is an immeasurable inferiority in the nervous systems of the Invertebrate com pared with even the lowest of the Vertebrate series of animals.
From all this it results, agreeably to the principle of which we are now showing the application, that the Invertebrate ought to have a much smaller capacity of producing caloric than even the cold-blooded Vertebrate animals; and this is exactly as we found matters to be by direct experiment in regard to the tempera tore of the different classes, the results of which have been already stated. The comparison might be carried out in regard to Insects and the Mollusca, which present some appreciable differences. If attention were confined solely to the structure of the greater number of the organs of nutrition, which are much more largely developed in Molluscs, it might be inferred that they had a higher calorific power than Insects; but when we take into the account, 1st, the final result of the nutritive functions, the quality of the tissues, which in the Mollusc are much more loaded with watery fluid, by which they acquire a greater degree of softness and flaccidity, (whence the class has its name,) whilst in the Insect they are, on the contrary, as remarkably dry and firm ;-2d, when the most general mode of respiration is compared in the two divisions, it being in the Insect aerial, in the Mollusc aquatic ; 3d and lastly, when we glance on the one hand and on the other at the state of the nervous system, and observe how much less perfectly this is developed in most of the Alolluscs than in the Insect, it is impossible not to perceive that according to the principles influencing the production of heat, the 11ollusca must be inferior in this respect to Insects. This is indeed the result of obser vations of all kinds, however imperfect or limited these may have been, as we have seen above.