496 BRITISH FOSSIL REPTILES. 



a blanket of blubber or other superficial modification, in aid of the maintenance of a fixed 

 and high degree of blood-heat. 



There are conditions, it is true, in which a Reptile generates a higher degree of heat 

 than is usual, but they are not those accompanying any unusual or excessive muscular 

 work and waste ; they are attended with rest, not locomotion. The incubating Boa gives 

 to the hand that may be insinuated between the coils surrounding the eggs the 

 sensation of a warm-blooded animal. Valenciennes ' found, in the Reptile-house at 

 the Jardin des Plantes, when its temperature, in the month of May, was 28° (Centi- 

 grade), that the heat of the Python, between the folds and upon the eggs, was 41-5° (ib.); 

 so also the heat of the incubating snrface of the Bird may rise to 10 degrees (Centicr.) 

 above the ordinary temperature — higher in this passive state than it ever reaches during 

 flight. 



The organic condition which determines the hot-blooded or cold-blooded nature of a 

 volant Vertebrate is the separation or the commingling of the arterial and venous bloods 

 in the course of their respective circnlations. From the demonstrated absence of any 

 heat-retaining covering of the skin in Pferosawria — the kind and amount of negative 

 evidence hereon being decisive — I infer that the black and red sanguineous streams were 

 mixed by intercommunication of the aortic trunks of the right and left ventricles, as in 

 the Crocodile.^ The plumose integument of Archaopteryx bespeaks the separation, not 

 only of the pulmonic and systemic ventricles, but of the arterial trunks thence arising ; it 

 was, consequently, hot-blooded, not because it coidd exert the muscular force required to 

 sustain itself in the air. The- all-important condition of the circulating system Las wide 

 correlations, not only with the extensive superficies acting upon the surrounding medium, 

 and being reacted upon thereby, but with a rapid and uninterrupted respiration, with an 

 advanced status of the nervous system, especially the brain, involving higher intelligence 

 and more lively and varied instincts, especially the parental. In the organic character 

 determining temperature, breathing, and psychical phenomena of life, Birds agree with 

 Mammals and differ from Reptiles. 



Birds agree with Implacental Mammals {LyencepUala) in the development, by the 

 embryo, of a vascular allantois devoid of villi for placental connection.^ They agree with 

 the same Mammals and diff'er from Reptiles in the transversely and deeply folded cere- 

 bellum, and in the larger proportion of that and of the cerebrum to the optic loljcs. Birds 



1 "Faites pendant I'incubation d'une femelle du Python a deux raies {Python bivittutus, Kuhl)," &c. 

 'Cotnptes rendus de I'Acad. des Sciences,' Paris, 19 Juillet, 1841. Something akin to this occurs in the 

 development of the generative elements in plants. 



= ' Anat. of Vertebrates,' i, pp. .510— ,512, figs. 339, 3-40. 



■5 This character is affirmed to be " of extreme importance, and to define Birds and Reptiles, as a whole, 

 very sharply from Mammals." — Prof. Huxley 'On the Classification of Birds,' loc. cit., p. 4IG. But, then, 

 the emphatic assertion comes from a writer on Elementary Physiology, who infers tlie blood of the Ptero- 

 sauria to have been hot because they were able to sustain themselves in air ! 



