24 THE BEGINNINGS OF LIFE. 



the animaFs nature and the necessities of its existence 

 compel it to manifest. Animals display, in varying 

 proportions, three principal modes of vital activity 

 which testify to the continual liberation of force within 

 them : (i) they appear to produce heat; (2) they move, 

 by reason of the contractility of certain tissues ; and 

 (3) they display certain nervous phenomena. 



i. Very many animals constantly maintain them- 

 selves at a temperature above that of the medium in 

 which they live; this being more especially the case 

 with the so-called warm-blooded animals amongst which 

 birds are most remarkable for the very great difference 

 existing between their temperature and that of the air. 

 The cause of this difference in temperature between 

 the animal and its medium has been variously explained 

 at different times. It was believed by Galen that heat 

 was actually produced de novo in the left ventricle of 

 the heart ; and even John Hunter thought that the pro- 

 duction of animal heat depended upon a special vital 

 force or principle, which was able not only to produce 

 but actually to destroy heat. Others and that even 

 in comparatively recent times have striven to prove 

 that some principle resident in the nervous system was 

 capable of giving rise to animal heat. The true theo- 

 ries on this subject, however, may be said to date as 

 far back as the close of the eighteenth century, and to 

 have commenced with the brilliant discoveries of Lavoi- 

 sier. Speaking of his researches, M. Gavarret says 1 : 



1 Loc. cit. p. 99. 



