10 CORRELATION OF VITAL 



maintained that these so-called vital forces were evolved within 

 the living bodies of plants and of the lower animals by the 

 transformation of the hght, heat and chemical action obtained 

 from without ; which forces were given back to the external world 

 again, either during the life of the living beings or after their 

 death in terms of motion and heat, and also, to a slight extent, in 

 the form of light and electricity. This has been found to hold 

 good, as we shall see later on, for plants and the lowest animals 

 and also for the initial changes in higher animals, though all the 

 later vital manifestations of the latter are dependent almost entirely 

 upon the redistribution of the forces locked up in the organic 

 substances which constitute their food, and to the various chemical 

 changes taking place within their bodies at the expense of their 

 own tissues. 



We have thus to do with a fundamental distinction between the 

 nutritive processes of plants and animals, for, as pointed out by 

 Gavarret in his interesting work " Phenomenes Physiques de la 

 Vie " (1869), most of the physical force which, in the form of light 

 and heat, impinges upon a plant, is consumed therein {travail 

 interieur). It is stored up as potential force in the complex organic 

 substances entering into the composition of the plant ; these 

 organic substances being produced (under the influence of the 

 already existing living tissues) by the action of the above-mentioned 

 physical forces upon the not-living constituents of the earth, air, 

 and water by which the plant is surrounded. The animal, on the 

 contrary, liberating and using the forces which have been stored 

 up by the plant — after assimilating a part of its substance in the 

 form of food — expends them in the production of that travail 

 exterieur which the animal's nature and the necessities of its 

 existence compel it to manifest. 



Now, as we aU know, animals display, in varying proportions, 

 three principal modes of ' vital ' activity which testify to the con- 

 tinual 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. Let us 

 briefly consider each of these manifestations. 



I. Very many animals constantly maintain themselves at a tem- 

 perature 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 



