These divisions are, first, the most comprehensive groups, the primary divisions, called branches by some, types by others, and divided by some naturalists into so-called sub-types, meaning only a more limited circumscription of the same kind of group; next we have classes, and these also have been divided into sub-classes, then orders and sub-orders, families, sub-families, and tribes; then genera, species, and varieties.  With reference to the question, whether these groups really exist in Nature or are merely the expression of individual theories and opinions, it is worth while to study the works of the early naturalists, in order to trace the natural process by which scientific classification has been reached; for in this, as in other departments of learning, practice has always preceded theory.  We do the thing before we understand why we do it:  speech precedes grammar, reason precedes logic; and so a division of animals into groups, upon an instinctive perception of their differences, has preceded all our scientific creeds and doctrines.  Let us, therefore, proceed to examine the meaning of these names as adopted by naturalists.

When Cuvier proposed his four primary divisions of the animal kingdom, he added his argument for their adoption,—­because, he said, they are constructed on four different plans.  All the progress in our science since his time confirms this result; and I shall attempt to show that there are really four, and only four, such structural ideas at the foundation of the animal kingdom, and that all animals are included under one or another of them.  But it does not follow, that, because we have arrived at a sound principle, we are therefore unerring in our practice.  From ignorance we may misplace animals, and include them under the wrong division.  This is a mistake, however, which a better insight into their organization rectifies; and experience constantly proves, that, whenever the structure of an animal is perfectly understood, there is no hesitation as to the head under which it belongs.  We may consequently test the merits of these four primary groups on the evidence furnished by investigation.  It has already been seen that these plans may be presented in the most abstract manner without any reference to special animals. Radiation expresses in one word the idea on which the lowest of these types is based.  In Radiates we have no prominent bilateral symmetry, as in all other animals, but an all-sided symmetry, in which there is no right and left, no anterior and posterior extremity, no above and below.  They are spheroidal bodies; yet, though many of them remind us of a sphere, they are by no means to be compared to a mathematical sphere, but rather to an organic sphere, so loaded with life, as it were, as to produce an infinite variety of radiate symmetry.  The whole organization is arranged around a centre toward which all the parts converge, or, in a reverse sense, from which all the parts radiate.