22 COMPARATIVE ZOOLOGY. 
botanists and zoologists; for they have characters that 
lead on the one side to plants, and on the other to ani- 
mals. To solve the difficulty, some eminent naturalists, 
as Hiickel and Owen, propose a fourth “ kingdom,” to re- 
ceive those living beings which are organic, but not dis- 
tinctively vegetal or animal. But a greater difficulty arises 
in attempting to fix its precise limits. 
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.”° 
In general, we may say that a rooted organism, retaining 
carbon and exhaling oxygen, feeding on mineral matter 
by absorption, and having cellulose tissues, is a plant; that 
an irritable or locomotive organism, retaining oxygen and 
exhaling carbonic acid, feeding on organic matter by a 
mouth and stomach, and having albuminous tissues, is an 
animal. But Nature knows no such line of demarkation ; 
for it is bridged at numerous points. 
(1) 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 
