CHAP. I.] OF A NERVOUS SYSTEM. 5 



' ephemeromorphs,' we are compelled to believe that 

 such passages from the one mode of molecular composition 

 and activity to the other, may be determined without any 

 great difficulty by internal chemico-nutritive changes, 

 whether these latter have or have not been in part induced 

 by external influences. Such transitions from vegetal to 

 animal modes of life, or the reverse, are regarded by the 

 writer as comparable with some well-known metamor- 

 phoses of form and nature amongst simpler kinds of matter.* 



It is certain, as Prof. Graham showed, that one and the 

 same saline substance may exist with its molecules now 

 in the crystalloid and now in the colloidal mode of aggre- 

 gation, according to the different influences under which 

 it has been produced, or to which it has been afterwards 

 subjected. This > for instance, is the case with silica, with 

 the sesquioxides of chromium and iron, and with other 

 mineral substances. On the contrary, it is also known that 

 certain typical colloids may, under some conditions, be 

 converted into crystalloids. 



Again, transformations of a similar order, though of 

 different degrees of complexity, are met with amongst 

 saline and elementary substances, when these assume 

 different ' allotropic' conditions. Well known illustrations 

 of this kind of metamorphosis are met with in the dif- 

 ferent interchangeable states of carbon, of phosphorus, 

 and of sulphur. The passage from one to the other 

 allotropic state amongst these elementary substances may 

 take place either with difficulty or with comparative readi- 

 ness, though the ease and celerity with which analogous 

 transformations are effected in the case of certain saline 

 substances is still more interesting in its bearing upon the 

 transformations of simple living units. No better instance 



* "Beginnings of Life," vol. fi. pp. 38, 55, 82. 



