SOME MOOT POINTS IN NATURAL HISTORY. 253 



tion of the bodies of the opposite organisms in either case. 

 If there exists one substance which might be thought to be 

 the exclusive property of plants, it is certainly the matter 

 which imparts the green colour to their tissues, and which is 

 so widely diffused through nature's domain. Yet this identi- 

 cal substance, or chlorophyll, as it is named, is found enter- 

 ing in the most intimate manner into the composition of 

 many true animals, and is thus duly manufactured or se- 

 creted by animal tissues. The common fresh-water polype 

 or hydra (Fig. 38), and many animalcules, are thus coloured 



FIG. 38. Hydrae, or fresh-water polypes, showing young budding from the 

 mature forms. 



green with the identical substance which we behold in the 

 leaves of plants. Starches and sugars, long regarded as the 

 exclusive products of plant life, are now well ascertained to 

 be manufactured by even the highest animals. And a 

 starchy substance known as cellulose, found entering in the 

 most intimate manner into the composition of plant-cells and 

 plant-tissues generally, has been noted to occur in large pro- 

 portions in the outer layer or covering of those sac-like 

 animals the sea-squirts, already alluded to. It would seem, 

 in fact, as if the animal form, like a dishonest manufacturer, 

 had infringed the patent rights of the plant, and had thus 

 produced substances which in bygone days were esteemed 

 the sole and exclusive products of the plant-economy. 



It may be urged, however, that the common and well- 



