There is not a less remarkable contrast between these two groups of birds in point of calorific power, than between the two groups of Mammalia already mentioned ; but the zoological or external characters which dis tinguish them in the present instance are not of the same kind. The state of the eyes does not apply here, for all Birds are diselosed with their eyes unsealed. They also all come into the world with the luaus artcriostis closed or nearly so,—a circumstance which might have been predicated, or inferred from analogy. Yet the young of Birds in the power of producing heat present diversities no less remarkable than are observed among the young of the Mammalia. The separation of the two kinds of blood con sequently is not the only condition which influences the production of heat; but all that modifies the blood on the one hand, and the nervous system on time other, as we have had occasion to observe in a previous part of this paper. Now it happens that we have an op portunity of applying this principle in a very particular manner in the instance of the two groups of Birds that engage us, and that differ so essentially in their powers of engender ing caloric. In the one and in the other we observe the same difference in the state of the general strength which we have observed in the corresponding groups of the Mammalia. In the one which cools rapidly, there is the same state of weakness, of general impotency ; in the other the young are in a condition to walk, and in a certain sense to shift for themselves as soon as they have escaped from the shell.
We perceive then in the first place, that the nervous system is much less energetic in the former than in the latter group ; and in the second place, that the digestive powers are in an equal degree inferior in strength ; for they are not only unable to take food of themselves from muscular incapacity, but also from the lack of the requisite instinct, and, farther, from their digestive organs not being in a condition to elaborate food to any extent. It is on this last account that the parents supply their young with food which has suffered maceration in their own crops, or has even in their stomachs undergone a kind of incipient or partial solu tion ; or otherwise the parents have the instinct to select such articles as are easiest of digestion, and best fitted for the weakly state of the digestive organs of their progeny. We have already observed that a detect in the powers of digestion implies a corresponding imperfection in the blood. Whence we must conclude by analogy that the blood in the birds of the first group is inferior in quality to that of the birds of the second group. We consequently still find the two general conditions which regulate the production of heat throughout the animal kingdom--the state of the blood, the state of the nervous system.
The same principles are applicable to the first period in the existence of all animals, without distinction of groups, as compared with adults. On the one hand we have ascertained that all without exception have a temperature lower than that of their parents ; on the other, nothing can be more manifest than their inferi ority with reference to the energy of the nervous system. And more attentive and extensive ex amination shows that this extends in like man ner to the digestive functions, and consequently to those of nutrition generally.
Let us first turn our eyes to the Mammalia. All of these are evidently inferior in this respect to the adult. This is proclaimed in the distin guishing character of the class : the females are provided with glands for the purpose of prepa ring a food appropriate to the state of weakness of their young. The state of the mouth of the young is a sufficient index of the defective power of the digestive organs ; the jaws are either wholly or partially without teeth. The softness, delicacy, paleness of colour, and insi pidity of the tissues of young Mammalia, com plete the evidence of the imperfect elaboration of the nutrient juices. If, therefore, the first and last products of the nutritive functions are in an inferior condition, can we suppose that the intermediate product, the blood, will not participate in this inferiority? We have already shown in what this consists among the Birds of the first group. With regard to the second, the
general considerations relative to the difference of the tissues is equally applicable to them, and these considerations possess a high value. When very young warm-blooded animals, with out any exception, are compared in this respect to the cold-blooded Vertebrata, we perceive a great analogy in their component tissues, which are softer and less savoury than among the adults of warm-blooded animals. It is thus that we can account for a striking anomaly in the nervous system of young warm-blooded animals, especially Mammalia. Their nervous system, particularly the encephalon, bears a higher proportionate ratio to the whole body than it does in the adult; but the softness and the other characters of the tissue of this organ in early life cause it to approximate in a re markable manner in appearance and character to the same tissue in the cold-blooded Verte brata. If, therefore, the relative volume predo minate in early life, one of the conditions favourable to calorification, the inferiority in respect of tissue counterbalances this advan tage, and is only compatible with very inferior manifestations of energy.
It is obvious then that there is a universally pervading analogy between warm-blooded ani mals in the first stages of their existence and adult cold-blooded Vertebrata, and that the pa rallel holds good, not merely with reference to their inferior power of producing heat, but also with regard to the functions of nutrition gene rally and the functions of the nervous system. There is one upon which it is highly ne cessary to insist, inasmuch as it is of the greatest importance, both theoretically and practically; it is this : that the analogy in the direction in dicated is by so much the more remarkable as the warm-blooded animal is born with charac ters which distinguish it more strikingly from those it possesses when arrived at maturity. If it is born with the eyes closed, or without fur or feathers, instead of with the eyes open and the body covered with a fur coat or a thick down, it is because the creature comes into the world less perfectly developed in every respect, and the whole economy is more closely allied to that of inferior orders. This, in other words, is as much as to say that the creature is born at a period relatively precocious, or in a more imperfect condition. Whence it may be in ferred that those warm-blooded animals which are born at a period short of the ordinary term of utero-gestation among the more perfect spe cies, will present a more marked analogy with the cold-blooded tribes. Man himself will form no exception to this rule, which must be quite general. The verification of this law has been completed by the phy siological experiments of the writer. A child born at the seventh month, perfectly healthy, and which had come into the world with so little difficulty that the accoucheur could not be fetched in time to re ceive it, had been well clothed near a good fire when the temperature was taken at the axilla. This was found no higher than 32° c. (under 90° F.). Now we have seen that the mean of the temperature of ten children born at the full time was 34°,75 c. (94°,5 F.); the tempera ture in no case descending lower than 34° (94° F.), and ranging between this and 35°,5 c. (96° F.). Let it be observed that at the seventh month the membrana pupillaris no longer exists ; the infant has, therefore, at this epoch of its development, the essential charac ters of warm-blooded animals capahle of sup porting a high temperature when that of the surrounding atmosphere is mild. But if it were entering the world some considerable time before the disappearance of the pupillary mem brane, it would be in a condition analogous to the Mammalia which are born with their eyes shut ; it would no longer be in a condition to maintain an elevated temperature, and without doubt would lose heat precisely as they do without precautions to the contrary.