2't6 On tlic Pi'oduclion of Heat and Light hy Animals, 



natural system of the animal creation, in all its generality, the 

 author wishes it to be understood that the views he has ex- 

 plained above are in no degree necessarily dependent on Mr. 

 Macleay's views of natural distribution, which, on their part, 

 are equally independent of the author's views of the distribu- 

 tion of the powers of producing heat and light. At the same 

 time the demonstrable truth of Mr. Macleay's inductions, fur- 

 nishes, when those inductions are pursued to all their conse- 

 quences, a strong a priori case in favour of the author's ; and 

 conversely, if the author's shall be found correct, a strong 

 confirmation of Mr. Macleay's will be afforded; because, — 

 while that zoologist infers the natural distribution of animals 

 from the totality of their structure and its variations, — the 

 author virtually infers their natural distribution from one or 

 two particular functions or points of structure ; and the ar- 

 rangements inferible respectively from these methods, if they 

 are both true, should exactly coincide ; so far, at least, as the 

 arrangement inferred by the latter method professes to be 

 complete. An examination of this subject will form part of a 

 subsequent communication. 



Finally, the author having been obliged to reserve for the 

 Memoir itself, on account of the detailed discussion which 

 they require, the proofs of the exchange of a certain degree 

 of animal heat for an equivalent degree of animal light, and 

 vice versdf as we proceed in the zoological series in either di- 

 rection from the maximum of either power, towards its mini- 

 mum and the maximum of the other, on which the demon- 

 stration of the law here announced depends, is desirous that 

 naturalists and natural philosophers should receive the an- 

 nouncement, — not as a law alleged to be ascertained — but 

 merely as a prediction hereafter to be verified. To the dis- 

 proof or verification of this, nothing will tend so greatly as a 

 series of exact researches on the vital heat of animals of va- 

 rious classes, especially of those which appear to lead from 

 one group to the other ; as the Penguins among Birds, which, 

 as Mr. Macleay has shown*, evidently approximate to the 

 Reptiles; the Cetacea among Mammalia; and the viviparous 

 Sharks, which, through the former group, connect the latter 

 with Fishes. Minute researches are also wanting on the tem- 

 perature of the Reptilia, and more particularly on that of the 

 Chelonian reptiles. But for these purposes, and especially for 

 that of ascertaining the difference of temperature between ani- 

 mals of the same or of contiguous groups, and for determin- 

 ing whether animals not hitherto regarded as enjoying vital 



* Hor. E?ttomol., p. 264. 



