2O8 GENERAL PRINCIPLES OF ZOOLOGY. 



realms of nature these belong. As such may be mentioned 

 the iny.vomycctcs and many flagellata. 



Distinction on a Physiological Basis. If one wishes 

 to discover sharp distinctions between animals and plants, 



he may take into consideration on the 

 one side physiological, on the other mor- 

 phological, characters. Starting from the 

 physiological point of view, Linnaeus 

 ascribed to plants only the capacity of 

 reproduction and nutrition, but to ani- 

 mals the power of motion and sensation 

 in addition to these. However, we 

 know that vegetable protoplasm, just as 

 well as animal, is irritable and is capable 

 FIG. .,.-*/*'./(/". of movement, as is shown by the active 

 r^lT^ter^uraV^'scu- movements of the lower alg<z , the great 



sensitiveness of the Mimosa, and other 

 plants ; but further, we know that even many of the more 

 highly organized animals, e.g. crabs (Fig. 109), lose the 

 power of locomotion and become fixed, and many fixed 

 forms, e.g. the sponges (Fig. Si), even under the closest 

 examination appear immovable and unaffected by stimula- 

 tion ; thus we are led to abandon the idea that the so- 

 called animal functions are to be regarded as accurate dis- 

 tinctions. 



Difference in Metabolism not a Safe Criterion.- 

 Even the difference in metabolism is by no means sufficient. 

 Every plant has a double exchange of material. In its 

 movements and other vital functions the vegetable proto- 

 plasm produces carbonic acid and consumes oxygen ; at 

 the same time, there goes on here, under the influence of 

 sunlight and of chlorophyl, the reduction of carbonic acid, 

 and the giving off of oxygen. In chlorophyl-containing 

 plants, during the day, the reducing process preponderates 

 so considerably that there is evident, as the final result, the 

 giving off of a greater quantity of oxygen, and only at 

 night, when the reducing process becomes interrupted on 

 account of the lack of sunlight, does the production of car- 



