The ultimate object which the zoologist has in view, in the employment of the preceding methods of investigation, is to complete the History of Species. For the full accomplishment, however, of this end, it is not only necessary to acquire a knowledge of their structure and functions, but likewise of all their mutual relations. This last task can only be executed, by calling to our aid the principles of arrangement, and by distributing animals into divisions or classes, according to the characters which they exhibit. Attempts of this kind have been made by numerous observers; and the various systems which have been proposed, differing from one another in the characters employed, and the divisions recognised, intimate very plainly the difficulties inseparable from the subject.

The methodical investigation and distribution of Animals, would be comparatively easy, if the forms and modifications of the different systems of organs exhibited constant mutual relations. Thus, if we consider the organs of any system to be in their most perfect state, when they admit into their structure the greatest variety of combinations, and execute the greatest number of motions or functions, does it happen, that, when we have discovered in any species, one system of organs in its most perfect state, all the other systems may be expected to be in the same condition.