666 



ANIMAL HEAT. 



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 defect 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 point 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 physiological 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 



