ORIGIN AND EVOLUTION OF LIFE 5"
��Physical environment, succession, reversal and alternation of habi-' tat zones,
Individual development, succession, reversal and alternation of adaptive phases.
Chromatin evolution, addition of the determiners of new adapta- tions while preserving the determiners of old adaptations,
Succession of life environments.
��Yet it must be the similarity of these internal physico-chemical energies of protoplasm and the similarity in the mechanics of motion, of offense and defense, which underlies the law of convergence or par- allelism in adaptation, namely, the production of externally similar forms in adaptation to externally similar natural forces, a law which escaped the keen observation of Huxley^ in his remarkable analysis of the modes of vertebrate evolution published in 1880.
The whole process of motor adaptation in the vertebrates, whether the fishes, amphibians, reptiles, birds, or mammals, is the solution of a series of mechanical problems, adjustment to gravity, overcoming the resistance of water or air in the development of speed ; in the evolution of the limbs of creating levers, fulcra (joints) and pulleys. The fore and hind fins of fishes and the fore and hind feet of mammals evolve uniformly where they are hemodynamic and divergently where they are heterodynamic. This principle of homodynamy and heterodynamy applies to the body as a whole and to every one of its parts, according to two laws: first, that each individual part has its own mechanical evolu- tion, and, second, that the same mechanical problem is generally solved on the same principle. This we observe is invariably the ideal prin- ciple, for unlike man nature wastes little time on inferior inventions but inmiediately proceeds to superior inventions.
The three mechanical problems of existence in the water habitat are : first, overcoming the buoyancy of water either by weighting down, increasing the gravity of the body, or by the development of special gravitating organs which enable animals to rise and descend in this medium; second, the mechanical problem of overcoming the resistance of water in rapid motion which is accomplished by means of warped surfaces and well-designed entrant and reentrant angles of the body similar to those of the fastest modem yachts; third, the problem of propulsion of the body, which is accomplished, first, by sinuous motion of the entire body terminating in tlie powerful propulsion of the tail fin, secondly, by supplementary action of the four lateral fins, third, by the horizontal steering of the body by means of the median system of fins.
B Huxley, T. H., * ' On the Application of the Laws of Evolution to the Ar- rangement of the Vertebrata, and more Particularly of the Mammalia," Troc, Scientific Meetings of the Zoological Sac. of London for the year 1880, pp. 649- 662. (Bead December 14, 1880.)