352 ANIMAL NATURE OP DIATOMEjE. 



plants all movements are determined by the physical or 

 chemical influence of external agents. In animals, on 

 the other hand, the laws of inorganic nature are not suf- 

 ficient to explain the connection (otherwise more mediate 

 and remote) between the external impression and the 

 organic movement. Which distinction, besides being 

 relative to the degree of our knowledge, has, moreover, 

 a value limited entirely to the coarser phenomena, not 

 being applicable to those of nutrition and growth ; for all 

 these are accomplished by determinate movements that 

 cannot be explained by merely physical and chemical 

 laws. " If nature," exclaims Humboldt, " had endowed 

 us with microscopic powers of sight, and if the integu- 

 ments of plants were transparent, the vegetable kingdom 

 would by no means present that aspect of immobility and 

 repose under which it appears to our senses." And the 

 same may be said of the instruments ; for the muscles of 

 higher animals very soon disappear when we examine 

 more simple or smaller animals, nor certainly can we 

 deny animality to those minute Infusoria in which we 

 are unable to distinguish either muscles or any other 

 distinct organs. 



Having then ascertained the insufficiency of these two 

 principal characters, it became necessary to look for 

 others, not in the simple manifestations of a supposed 

 subjective property, but in the objective qualities them- 

 selves. And these not in their external form, for we 

 know not the limits of its variety, but in the internal 

 organisation, in the state of the more remote organic 

 elements, in their origin and formation, or in the chemical 

 materials of which they are constituted. This dilficult 

 inquiry is assisted by recent discoveries, through which 

 science is now put in possession of a most important 

 truth, — that within the superior organic type, as well 

 in the vegetable as in the animal kingdom, there is 

 included, so to speak, in a summary manner, the history 

 of the lower, which present in a permanent form their 

 various intermediate states : that the same histological and 



