Page:Popular Science Monthly Volume 4.djvu/635

Rh radiating circulatory tubes to the digestive cavity. The circulatory fluid is chyme, and not chyle, as it is in the Articulata and Mollusca. The author describes four species, and distinguishes a new genus—Nemopsis.

Ever since his arrival in America, Agassiz had been collecting material for a series of "Contributions to the Natural History of the United States." In 1857 appeared two volumes of these contributions; the first containing an "Essay on Classification" and the history of the North American Testudinata; the second on "the Embryology of the Turtle."

In the essay on Classification, Agassiz affirms that Nature is but the expression of the thought of the Creator, and that a true classification will be found to be but an unfolding of the plan of creation, as expressed in living realities; that these realities do not exist in consequence of the continued agency of physical causes, but appear successively by the immediate intervention of the Creator. We find in Nature a progressive series, from lower to higher forms; but it is not a uniform progress for the animal kingdom as a whole; neither is it a linear progress for the branches or classes, but a progress in which each type has usually been introduced by the creation of species belonging to one of its higher groups, for the earliest representatives of a class do not always seem to be the lowest. Yet, notwithstanding these downward steps, the progress has continually tended toward the production of higher and higher types, culminating at last in Man.

We find a parallelism between the geological succession of animals and the embryonic growth of their living representatives, as well as a parallelism between the geological succession of animals and their relative rank. The earlier types of animals were synthetic or prophetic, foreshadowing a future group. Types likewise culminated and disappeared in past ages—a feature parallel with the fact that in embryological development parts fulfill their end and then disappear.

Regarding Nature as the embodiment of a certain divine plan, Agassiz sought for a classification that should be the expression of this plan. Accordingly he based his classification on the following divisions, which he deemed covered all the categories of relationship, as far as their structure is concerned: Branches, or types characterized by their plan of structure; of these, he admitted four, Vertebrates, Articulates, Molluscs, and Radiates. Classes, which are characterized by the mode of execution of the plan. Orders, by the degree of complication of structure. Families, by the form as determined by structure. Genera, by the details of execution; and Species, by the relation of the individuals to one another, and to the world in which they live.

In Part II., Agassiz believes that the Testudinata constitute an order among the class of reptiles; that their essential character lies not so much in the shield as in the special development of different regions of the body, thus giving them the highest rank in the class.