22 COMPARATIVE ZOOLOGY. 



The drift of modern research points to this : that there 

 are but two kingdoms of nature, the mineral and the or- 

 ganized, and these closely linked together; that the lat- 

 ter must be taken as one whole, from which two great 

 branches rise and diverge. " There is at bottom but one 

 life, which is the whole life of some creatures, arid the 

 common basis of the life of all ; a life of simplest moving 

 and feeling, of feeding and breathing, of producing its 

 kind and lasting its day: a life which, so far as we at 

 present know, has no need of such parts as we call organs. 

 Upon this general foundation are built up the manifold 

 special characters of animal and vegetable existence ; but 

 the tendency, the endeavor, so to speak, of the plant is 

 one, of the animal is another, and the unlikeness between 

 them widens the higher the building is carried up. As 

 we pass along the series of either [branch] from low to 

 high, the plant becomes more vegetative, the animal more 

 animal." 6 



Defining animals and plants by their prominent char- 

 acteristics, we may say that a living being which has cell- 

 walls of cellulose, and by deoxidation and synthesis of its 

 simple food-stuffs produces the complicated organic sub- 

 stances, is a plant ; while a living being which has albu- 

 minous tissues, and by oxidation and analysis reduces its 

 complicated food-stuffs to a simpler form, is an animal. 

 But both definitions are defective, including too many 

 forms, and excluding forms that properly belong to the 

 respective kingdoms. No definition is possible which 

 shall include all animals and exclude all plants, or vice 



(i) Origin. Both branches of the tree of life start alike : 

 the lowest of plants and animals, as Protococcus and Gre- 

 garina, consist of a single cell. In fact, the cycle of life 

 in all living beings, high or low, begins in a small, round 

 particle of matter in plants called an ovule, in animals 



