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 tlie 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 mollusca, 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 

 w^ill be best shown by arranging them in a circle. The entire organism 

 being moved through the nervous system, this has more recently received a 

 gre%t 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 Dumeril 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 nscs 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 lias 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, nntil 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 exjDel 

 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, trachea?, and lungs, those of animals. 

 With regard to nutrition, animals and plants are nourished by extraneous 

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