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, and 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 
versa. 
(l) Origin.— Both branches of the tree of life start alike: 
the lowest of plants and animals, as Protococcus and Ore- 
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 
