Page:Popular Science Monthly Volume 45.djvu/287

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Space will not permit us to further notice the special points elaborated by the author, the table of contents alone occupying two crowded pages. The work is divided into four parts: I. A general account of the life-history of salpa. II. The systematic affinity of salpa in its relation to the conditions of primitive pelagic life; the phylogeny of the Tunicata; and the ancestry of the Chordata. III. A critical discussion of my own observations and those of other writers, on the sexual and asexual development of salpa. IV. On the eyes and subneural gland of salpa, is by M. M. Metcalf, who, among other points claims, contrary to Buetschli, that the eye of salpa is not homologous with the eye of any other chordate animal.

The general reader and biologist will be especially interested in the views presented in Part II. Brooks speaks of the wonderful scarcity of pelagic life in the lagoons and landlocked waters of the Bahamas, and explains it by the theory that the surface life is eaten up by the animals at the bottom, every organism swept in by the tides and every larva born in the sounds being eaten up by the polyps, etc., at the bottom, the competition for food being so fierce. He maintains that early in the Cambrian period, or when life first began, it was pelagic, or confined to the surface. Gradually some of the pelagic forms, at first minute and simple, settled at the bottom, and such a primitive bottom fauna was similar to the lower Cambrian fauna. This bottom fauna at first entirely depended for food upon the pelagic life at or near the surface, there being no plant life yet in existence. This primitive bottom fauna was established around elevated areas in water deep enough to be beyond the influence of the shore. He claims that the great groups of Metazoa, or all animals above protozoans, were rapidly established from pelagic ancestors. This, it may be said in passing, is in direct opposition to the view generally entertained that the pelagic fauna is derived from the shoalwater or shore life.

After the establishment of the first bottom fauna competition swiftly arose, became very rigorous, and led to rapid evolution, and "life on the bottom introduced many new opportunities for divergent modification and for the perfecting of animals." The increase in size of the animals also increased the possibilities of variation, and led to the natural selection of those peculiarities which increased the efficiency of different organs, and thus proved an important factor in the evolution of complicated organisms; the new modes of life—what they were, the author does not state, but they must have been in great part the results of fixation at the bottom, together with the operation of currents, etc.—permitting the acquisition of protective shells, or hard, supporting skeletons. Life at the bottom also introduced the factor of competition between blood relations, the fiercest competitors of each kind of animal being its closest allies, "which having the same habits, living upon the same food, and avoiding enemies in the same way, are constantly striving to hold exclusive possession of all the essentials to their life." Thus the tendency of such bottom forms was to divergent evolution of the great types of animal life. Since then, the author claims, "evolution has resulted in the elaboration and divergent specialization of the types of structure which were already established, rather than in the production of new types." This is all very likely, and, to continue the train of reasoning, the next great step was the origin of land animals, terrestrial and fresh-water arthropods, and the third great step was the evolution of animals, arthropod and vertebrate, adapted for life in the air. We may suggest that it was the Lamarckian factors of profound and widespread changes in the environment, such as a transfer of the habitat of animals from the surface to the ocean bottom which tended to increase and diversify life forms, together with the use and disuse of organs resulting from enforced adaptation to the new conditions. After all this had begun there comes in the more passive factor of natural selection, subordinate, though constantly at work, which further promoted the elaboration and specialization of organic forms.

Letters of Asa Gray. Edited by Jane Loring Gray. In Two Volumes. Boston: Houghton, Mifflin & Co. Price, $4.

Dr. Gray was a delightful correspondent. He wrote with the easy manner and hearty tone that give letters then-highest charm. In telling distant friends what he