102 TlMEHRI. 
The causes of this difficulty, moreover, are easily seen, 
when we bear in mind that while, on the one hand, in the 
higher plants and animals there are certain chara6leris- 
tics, such as form, presence or absence of a digestive 
apparatus, power of motion, etc., which will serve easily 
to distinguish the animal from the plant, on the other 
hand, in the lowest types no corresponding differences in 
these chara6leristics prevail, the form of the lowest 
plants and animals being often identical, and the absence 
of a digestive apparatus and the power of motion being 
features equally marked in both groups. 
More than this, the green colouring matter or chloro- 
phyll \ so chara6leristic of the generality of plants, is quite 
absent from the great group of the Fungous plants such 
as the moulds and mushrooms — a group, moreover, that 
furnishes a very large proportion of forms allied to those 
in the debatable land ; while this colouring matter is 
present is many undoubted animal organisms such as the 
Fresh-water polypes, the Trumpet-animalcule, etc. 
Again, the substance cellulose that presents such a 
marked feature in the life of plants, forming the outer 
layer or covering of the ultimate units or cells of which 
the plant is composed, is yet absent from many of the 
lowest plants during a great portion of their existence, 
and must thus be regarded as not being an essential to 
plant organisation, though highly charafteristic of it ; 
while, among a great group of highly organised animals 
known as Ascidians or Sea-squirts, this same cellulose is 
present in their outer covering or test, and must thus be 
regarded as a possible, though by no means chara6leristic, 
feature of animal organisation. And again, while plants 
are able to build up, and store away in their various parts, 
