8 



HUTCHINSON'S POPULAR BOTANY 



of distinction exist; and to prepare a definition either of an animal or a 

 plant, which shall be at once sufficiently full and sufficiently exclusive, 

 is in the present state of onr knowledge impossible. Probably, indeed, the 

 line of demarcation between the simpler forms of the two kingdoms will 

 never be absolutely determined. 



Three important characteristics may, however, be said to distinguish 

 the higher animals viz., the power of locomotion, evident sensitiveness, and 

 the possession of a special digestive cavity for receiving solid food : just 

 as the absence of these characteristics will be found to distinguish the 

 higher plants ; though even here exceptions are not wanting. Thus, among 



the higher animals the oyster lacks 

 the power of locomotion, and the 

 tape-worm has neither sensitive- 

 ness nor a special digestive cavity ; 

 while among the higher plants 

 we find a power of locomotion in 

 the spermatozoids of Ferns, ex- 

 treme sensitiveness in the 

 Mimosas, and " a kind of external 

 stomach which digests solid food " 

 in the Pitcher-plants (figs. 18 and 

 19). The proposal gravely made 

 by a French savant to define an 

 animal as -im estomac servi par des 

 organes is, therefore, not to be 

 thought of; and the inadequacy 

 of the definition is more plainly 

 seen when we descend to the 

 lower forms of life. Here, not 

 only are locomotion and apparent 

 sensitiveness common among the 

 simpler water-plants, as Sphcerella 

 pluvialis * and Volvox globator (fig. 20), but the absence of a digestive 

 cavity is the rule rather than the exception in the lower animalcule 

 (Protozoa], of which the Amoeba and its immediate allies furnish good 

 illustrations. Indeed, we must ascend the zoonic scale as high as Vorti- 

 cella, the curious little Bell-animalcule (fig. 23), before we meet with even 

 the rudiments of a digestive apparatus. 



Now, any one who would understand the complex forms of Life, whether 

 in the Animal or Vegetable world, does well to begin low down in the 

 scale by studying Life in its simplest forms; and unicellular, or one-celled, 

 plants supply excellent examples for the purpose. Allusion was made a 

 moment ago to Sphcerella pluvialis, one of the simplest forms of vegetable 

 * Protococcus viridis of Thome ; Hcematococcus pluvialis of Flotow, Prantl, and Vines. 



Pholoby'i [A. Leal. 



FIG. 20. THE REVOLVING GLOBE 



( Volvox globator). 



Variously regarded as a plant and a simple animal. About 

 fifty of them in a row would measure one inch. 



