Appendix. 81 



With regard to food, there is a wide difference between animals and the 

 great majority of plants. Animals feed upon other organisms, or on organic 

 substances ; they cannot build up new tissues, nor repair the waste occasioned 

 by vital activity, with inorganic materials alone, although, of course, these 

 occur as constituents of the food (water, calcareous salts). Plants, on the 

 other hand, can feed exclusively upon inorganic substances, and in coiTclation 

 with this, some of their cells are provided with a peculiar green substance, 

 chlorophyll, which enables them, in the presence of light, to separate and 

 assimilate the carbon from the carbon-dioxide in the air. This power is not 

 possessed by animal ceUs, which are always destitute of chlorophyll* : on the 

 other hand, there are not a few plants {e.g. all the Fungi), from which it is also 

 absent. These cannot, therefore, assimilate carbon from the air, but, like 

 animals, must obtain it from organic substances. 



Finally, in contrast to plants, animals possess sensibility and the powei 

 of voluntary movement, which seems to be a mark of absolute distinction 

 between plants and the Metazoa. 



From the foregoing remarks, there can never be any doubt as to whether a, 

 given organism is a multicellular plant or a Metazoon, at all events, if a 

 careful examination has been made ; but it is otherwise with the unicellular 

 organisms. They cannot be considered in the light of the difEerences 

 characteristic of multicellular plants and animals, just enumerated : indeed, it 

 is impossible to speak of alimentary canal, of musculature, of nervous system, 

 or of systems of organs at all, in unicellular animals ; for the most part it is 

 difficult to say whether sensation and voluntary motion are exhibited or not ; 

 and the mode of feeding is no absolute test (c/., above). To divide the 

 unicellular organisms between the two kingdoms is, to a certain extent arbitrary, 

 but since chlorophyll is peculiar to plants, and never present in the Metazoa, 

 it is justifiable to regard all organisms containing this colom-ing matter as 

 plants : on the other hand, the numerous unicellular organisms which are 

 destitute of chlorophyll must not be regarded as animals without some further 

 demonstration : for it is undoubtedly wanting in many plants, and its absence is 

 not therefore adequate proof that the organism is an animal. Further, it 

 must be taken for granted that those organisms with a cellulose cell-wall 

 belong to the vegetable kingdom ; whilst, on the other hand, those which exhibit 

 in their protoplasm a differentiation recalling the cell-modification of the 

 Metazoa (the muscle-like protoplasm in the Infusoria), should be classed as 

 animals. All those, moreover, which ingest solid food are considered to be 

 animals ; but for many " animals " even this does not hold, and their location is 

 purely a matter of custom. 



* It has been stated that chlorophyll is present in some animals, but this supposed 

 chlorophyll has been proved either to be some other green colouring matter, or to 

 belong to Algie parasitic in the animal. 



