ANIMALS AND PLANTS 65 



regarded as sufficiently characteristic to distinguish them. But 

 numerous sensitive plants, the Venus fly trap for example and 

 many algae have the power of movement quite as well developed 

 as do many animals, while many animals, as for example the 

 sponges, are quiescent. Again it was thought that chlorophyll, 

 giving well-defined colors to plants was a definite and distinc- 

 tive feature. But many animals such as Euglena and many 

 other flagellates are similarly colored by chlorophyll. The 

 presence of chlorophyll indicates a power of manufacturing 

 starch and sugars, while plant cells generally have a definite 

 me mbrane of cellulose which is closely allied to starch in chem- 

 ical composition. Cellulose therefore was also regarded as a 

 specific plant characteristic. But again a number of animals 

 have the power of manufacturing cellulose; the Dinoflagellates 

 for example have a thick cellulose wall while some of the higher 

 animals notably the group known as the ascidians have well- 

 del ined tests of the same material. Still later it was maintained 

 th^.t plants do not eat solid food in the form of proteids while 

 animals do; but the Venus fly trap Dionaea not only moves and 

 catches but also digests insects of various kinds. On the other 

 hand a number of animals especially the unicellular ones, do not 

 take in solid food but manufacture it as do the plants through 

 the aid of chlorophyll. Similarly with every other distinctive 

 feature we find some exceptions on one side or the other showing 

 that physiological processes in nature are nowhere monopolized 

 by any one type of organisms. 



While it is impossible to draw a definite line between animals 

 anc plants it is possible nevertheless, through the sum of char- 

 actors to determine whether an organism is either plant or 

 animal, or some form of life intermediate between, them. For 

 the determination of a given questionable type it is necessary 

 to 1 ake into consideration not only form, movement, and mode 

 of nutrition but also the immediate relations. Thus Euglena 

 has many of the physiological characteristics of plants of which 

 the mode of nutrition is the most important; but it itself and a 

 nearly-related type, Chromulina flavicans has the power of both 

 holophytic and holozoic nutrition and can live in the dark on 



