Page:The Scientific Monthly vol. 3.djvu/516

 5IO THE SCIENTIFIC MONTHLY

parative anatomy long before the actual lines of evolutionary descent were determined through paleontology.

The form-evolution of the backboned animals, beginning with these pro-fishes of Cambrian and Pre-Cambrian time, extends over a period of 30,000,000 years. The supremely adaptable vertebrate body type begins to dominate the living world, overcoming one mechanical diffi- culty after another as it passes through the habitat zones of water, land and air. Motions necessary for the capture, storage and release of plant and animal energy continue to control the form of the body and of its appendages, but in the meantime the organism through mechan- ical and chemical means protects itself either offensively or defensively and also adapts itself to reproduce and protect its kind, according to Darwin's original conception of the struggle for existence as involving both the life of the individual and the life of its progeny. Among all defenseless forms speed is a prime necessity, while all heavily armored forms gradually abandon mobility. As among the Invertebrata, cal- cium carbonate and phosphate and various compounds of keratin and chitin are the chemical materials of armature. Locomotion, as distin- guished from that in all invertebrates, is in an elongate body stiffened by a central axis. The evolution of the skeletal supports (endoskele- tal) and limbs is generally from the center of the body (notochord) toward the periphery, the evolution of the defensive armature (exo- skeleton) is from the periphery toward the center. The defensive armature finally, through change of function, makes important contri- butions to the inner skeleton.

THE LAW OF CONTEUOENCE OR PABALLBLISM IN LOCOMOTOR, 0FFEN8IVK

AND DEFENSIVE ADAPTATIONS

Although the structural body type and mechanism of locomo- tion is profoundly diverse, there arise hundreds of adaptive parallels between the Vertebrata and the antecedent evolution of the Inverte- brata. The combined necessity for protection and locomotion brings about close parallels in body form between such primitive Silurian eurypterids as Bunodes and the vertebrate Ostracoderms, a superficial resemblance which has led Patten* to defend the view that the two groups are frenetically related.

The theoretic application of the fundamental law of action, reac- I

tion and interaction becomes increasingly difficult as adaptations mul- tiply and are superposed upon each other with the evolution of the four physico-chemical relations, as follows:

♦ Patten, Wm., ''The Evolution of the Vertebrates and their Kin," 8vo, P. Blakiston'B Son & Co., 1912, 486 pp., 309 figs. |

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