ANIMAL GRAFTS AND REGENERATIONS. 249 



having at bottom the same forces, the same tendencies, the 

 same aspirations as the more or less complex systems to 

 which it gives birth by a thousand associations and vari- 

 ous interweavings. "Nature's machines," says Leibnitz, 

 " are machines throughout, however small the part in them 

 one may take ; or, rather, the least part is in its turn an infi- 

 nite world, and one even that expresses, after its fashion, all 

 that there is in the rest of the universe. This surpasses 

 our conception, yet we know that it must be so, and all 

 that infinitely infinite variety is established in all its parts 

 by a sublime constructive wisdom more than infinite." * 



But what is in itself the vital force peculiar to these tiny 

 machines, the force that we observe maintaining itself in 

 the several parts of the organism, and restoring the voids 

 produced in the tissues ? what is the fundamental character, 

 the mark of life ? It is nutrition, that is to say, the fact, as 

 plain as it is inexplicable, of the continuous molecular re- 

 newal of organized substance. It is in the understanding 

 of the phenomena of nutrition, the " trophic " phenomena, 

 that the whole future of biology lies. We shall never grasp 

 the secret of the deepest and most essential vital actions, 

 until we shall comprehend the equations of the statics and 

 dynamics of those fleeting systems, restlessly passing 

 through cycles of change, which compose the anatomical 

 elements. 



Whatever future the knowledge of trophic phenomena 

 will bring with it, the conception of life won for us by nat- 

 ural philosophy opens from this time forward a new path 

 for investigations. It suggests the thought of examining 

 into the variations of physiological determinism, that is, of 

 studying the boundaries within which life moves, or, in other 

 words, the profound modifications of which organisms are 

 susceptible, whether from the point of view of the specific 



1 "Letter to Bossuet," Unedited Works, published by Toucher de 

 Careil, voL i., p. 276. 



