THE LIFK OF MATTEE. 423 



surface is accessible, tke incorporation of similar particles is possible 

 only l).v exterior juxtaposition,' and the ediiice increases only because 

 a new layer of stones has been added to those which were there before. 

 On the contrary, the body of tin animal is a mass eminently penetrable. 

 The cellular elements that compose it have forms that are more or less 

 rounded and flexible. Their contact is by no means perfect. They do 

 not have either the stift'ness or the precision of adjustment that the 

 crystalline particles have. Li([uids and gases from without can insin- 

 uate themselves and circulate within the meshes of this loose construc- 

 tion. Assimilation can therefore take place throughout its whole depth, 

 and the editice increases because each stone increases by itself. 



The secondary and eo mi no n place character of the process of intussus- 

 cept'ion. — The apparent opposition of these two processes is doubtless 

 diminished if we compare the simple mineral indixidual with the ele- 

 mentary living- unit, the crystalline particle with the protoplasmic 

 mass of a cell. AMthout carrying an analysis so far as this, it is yet 

 easy to see that apposition and intussusception are mechanical means 

 that living beings employ at one and the same time and combine accord- 

 ing to their necessities. The hard parts of the interior and exterior 

 skeleton increase, ])oth by interposition and superposition at once. It is 

 by the last method that bones increase in diameter, and that are formed 

 the shells of mollusks, the scales of reptiles and tishes, and the testa of 

 many radiate animals. In these organs, as in crystals, life and nutri- 

 tion occur at the surface. 



Apposition and intussusception are then secondary, mechanical 

 arrangements having relation to the ph^'sical characters of these bodies: 

 Solidity in the crystal, semifluidity in the cellular [)rotoplasm. If we 

 compare the inorganic liquid matter with the semifluid organized 

 matter, we recognize that the addition of substance is made in the same 

 manner in each, that is to say, by interposition. If we add a soluble 

 salt to a fluid, the molecules of the salt separate and interpose them- 

 selves between those of the fluid. There is, therefore, nothing espe- 

 cially mysterious or particularly vital about the process of intussus- 

 ception. Applied to fluid protoplasm it is mereh^ the diffusion that 

 ordinarily occurs in mixed li({uids. 



VII. 



GENERATION IN BUUTK BODIES AND LIVING BODIES— SPONTANEOUS 



GENERATION. 



We have not yet exhausted the analogies between a crystal and the 

 living being. The possession of a specitic form, the tendency to 

 reestablish it by redintegration and the existence of a sort of nutri- 

 tion do not suffice to constitute a complete similarity. It still lacks a 

 fundamental character, that of generation. Chaufl'ard, some time ago. 



