4 ZOOLOGY. 
according to which the organs are developed from the lowest to the highest 
classes which associates it with comparative anatomy and physiology; the 
latter compares individual animals with each other to determine the 
peculiarities of individual species, treating of their character and habits. 
The attempt has been frequently made to arrange animals in a regular 
scale from the lowest to the highest, under the impression that each.animal 
must have an equal, a higher, or a lower organization when compared with 
others. This might be the case if there were only one set of organs; but 
as there are many, an animal may have a simple organization in one, and a 
complicated one in another. Thus by their organs of locomotion and general 
economy, insects would be placed above mollusea, whilst the latter are 
allied to the higher orders by their circulation. One proposes to arrange 
animals in a series of parallel lines, whilst another thinks that their affinities 
will be best shown by arranging them in a circle. The entire organism 
being moved through the nervous system, this has more recently received a 
great share of attention; and although it has proved satisfactory to a 
certain extent, it is at times difficult to make safe deductions from variations 
in the details. Under these circumstances, there are to be found those who, 
like Duméril and Swainson, think that external characters are sufficient for 
the classification of animals, as it is through these that they are placed in 
communication with external nature. Blainville makes the external organs 
the basis of his twenty-five divisions; considering their position, the skin 
and its appendages, and the structure and uses of the limbs. 
There are many important affinities between plants and animals, as we 
have already mentioned in the introduction to Botany. One of the most 
important of these has been discovered in modern times, by means of the 
improved microscope. Thus it has been shown that the structure and 
growth, as well in animals as in plants, is due to cells. There are, besides, 
other points of similarity, which will be stated further on. 
It is still doubtful whether certain organisms belong to plants or animals, 
there being grounds upon each side of the question. Some animal pro- 
ductions, as corals, were at an early day regarded as plants ; whilst certain 
vegetable productions have, until a recent period, been considered animals, 
and indeed some are still considered such. These doubtful organisms are 
to be found chiefly among the low and minute forms which require a 
microscope for their investigation. In the case of sponges, strong argu- 
ments have been brought forward upon both sides, by acute observers who 
have examined them in a living state, for the sponge of commerce is a 
mere skeleton. 
At first view, animals and plants would seem to be sufficiently separated 
by the respiration ; as the former breathe and assimilate oxygen and expel 
carbonic acid gas, whilst in the latter this operation is reversed. This 
view of a contrary action is, however, not strictly correct, because in 
animals respiration must continue without intermission, whilst plants breathe 
inwards by day and outwards by night. Leaves and spiral vessels are the 
breathing organs of plants ; gills, tracheze, and lungs, those of animals. 
With regard to nutrition, animals and plants are nourished by extraneous 
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