THE SCOPE OF ZOOLOGY 7 



the formation of new living beings by the cutting off of small 

 portions, termed germs, from pre-existing living beings, which 

 then by growth come to resemble the beings from which they 

 took their origin. Only in this way are new living beings 

 formed ; new life can only arise in connection with pre-existing 

 life, and reproduction is only a special case of growth. It is a 

 marvellous thought to reflect that although living beings are 

 continually dying, i.e. being turned into dead matter which 

 neither moves nor grows, yet dead matter can only be con- 

 verted into a living matter into a part, that is, of a living 

 being through being eaten or assimilated by that being. 



An animal is defined, for the purposes of science, as a living 

 being which moves and which requires for its sustenance food 

 containing peculiarly complicated chemical compounds known 

 as prbteids; these in the majority of cases are taken in the 

 solid form and digested, i.e. melted inside the animal's body. 

 Plants, on the other hand, are defined as living beings which 

 can only take their food in the form of liquids and gases, but 

 which require only simple chemical compounds for their sus- 

 tenance, and which do not move except so far as movement 

 is involved in the process of growth. But in practice these 

 definitions are not rigidly adhered to. The reason for this 

 is that both plants and animals can be divided into groups, 

 the members of which exhibit the closest resemblance to one 

 another, and a particular living being is reckoned as animal or 

 plant according to the general characteristics of the group 

 to which it belongs, even should its own individual peculiarities 

 be in some respects irreconcilable with the general definitions 

 of animal or plant. Thus the sensitive plant droops its leaves 

 and shuts up its leaflets when touched, just as the snail draws in 

 its tentacles and retires into its shell when it is irritated ; and yet 

 the sensitive plant is reckoned a plant, because in all its char- 

 acters it closely agrees with the plants of the Pea order. The 

 carnivorous plants form hollow cups at the tips of their leaves, 

 which entrap insects by means of slanting hairs and other con- 

 trivances. These insects die and their bodies are digested, and 

 the resultant materials are absorbed by the surrounding tissues 

 of the plant which act like the walls of the stomach of an 

 animal. Nevertheless we reckon these carnivorous plants as 

 plants, because in all the details of their structure wood, 



