CHAPTER I. 



I. THE DISTINCTIVE CHARACTERS OF ANIMALS. 



The more complicated forms of the living things, the 

 general characters of which have now been discussed, appear 

 to be readily distinguishable into widely-sejDarated groups, 

 animals, and plants. The latter have no power of locomo- 

 tion, and only rarely exhibit any distinct movement of their 

 parts w T hen these are irritated, mechanically or otherwise. 

 They are devoid of any digestive cavity; and the matters 

 which serve as their nutriment are absorbed in the gaseous 

 and fluid state. Ordinary animals, on the contrary, not only 

 possess conspicuous locomotive activity, but their parts 

 readily alter their form or position when irritated. Their 

 nutriment, consisting of other animals or of plants, is taken 

 in the solid form into a digestive cavity. 



But even without descending to the very lowest forms of 

 animals and plants, w T e meet wdth facts which weaken the 

 force of these apparently broad distinctions. Among animals, 

 a coral or an oyster is as incapable of locomotion as an oak ; 

 and a tape-worm feeds by imbibition and not by the ingestion 

 of solid matter. On the other hand, the Sensitive-Plant and 

 the Sundew exhibit movements on irritation, and the recent 

 observatious of Mr. Darwin and others leave little doubt that 

 the so-called " insectivorous plants " really digest and assimi- 

 late the nutritive matters contained in the living animals 

 which they catch and destroy. All the higher animals are 

 dependent for the protein compounds which they contain 

 upon other animals or upon plants. They are unable to man- 

 ufacture protein out of simpler substances; and, although 

 positive proof is wanting that this incapacity extends to all 

 animals, it may safely be assumed to exist in all those forms 

 of animal life w T hich take in solid nutriment, or which live 

 parasitically on other animals or plants, in situations in which 

 they are provided with abundant supplies of protein in a 

 dissolved state. 



