182 FOOTNOTES FROM 



mate test before which every doubt and difficulty usually 

 vanish, is of no assistance to us in their determination ; 

 for in their elementary composition they are identical 

 with some of the lowest members of the animal kingdom. 

 In these primitive plants and animals, we may fairly 

 enough conclude that the animal and vegetable kingdoms 

 pass into each other ; they form the one common base or 

 point from which these two systems of life start, to recede 

 so widely from each other in the large and complicated 

 organizations which stand at the head of both. " From 

 man to the primary animal and vegetable cell," Schmidt 

 justly observes, " there exists no gap in the realization 

 of a general idea upon which nature as a whole is based. 

 There is no abrupt transition from one kingdom to 

 another, but an insensible gradation. Thus the embryo 

 germ of an alga or sea-weed is identical, in elementary 

 composition and form, with that of a medusa or ascidia ; 

 in the former we have the higher stage of development 

 of the plant, in the latter the simpler form of the 

 animal." 



The forms which the diatomaceaB assume are exceed- 

 ingly varied and beautiful. Most of them, as already 

 mentioned, are invested with a very thin transparent 

 glass-like pellicle, engraved with median lines and trans- 

 verse striae, the patterns of which are wonderfully con- 

 stant in the same species, and afford admirable tests for 

 the general excellence of the object-glass of the micro- 

 scope ; the distance between the different markings being 

 often the 35 5ooth P ar t of an inch, and some, it is even said, 

 being only the yssoooth of an inch separate, requiring for 

 their distinct determination a magnifying power of twelve 



