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��THE SCIENTIFIC MONTHLY

��The terrestrial and aerial evolution of the four-limbed types (Tetra- poda) is designed chiefly to overcome the resistance of gravity and in a less degree the resistance of the atmosphere through which the body moves. When the aerial stage evolves, with increasing speed the re* sistance of the air becomes only slightly less than that of the water in the fish stage, and the warped surfaces, the entrant and reentrant angles evolved by the flying body are similar to those of the rapidly moving fishes.

��BRAKCHINO OB DIVEHOENCE OF FOBK, THE LAW OF ADAPTIVE RADIATION

In general the law of divergence of form, perceived by Lamarck and rediscovered by Darwin, has been expanded by Osbom into the modern law of adaptive radiation, which expresses the differentiation of animal form radiating in every direction in response to the necessities of the quest for nourishment and the development of new forms of motion in the different habitat zones. Divergence is constantly giving rise to differences in structure, while convergence is constantly giving rise to resemblances of structure. In contrast with this divergent principle is the convergence brought about by the similarity above described of the physico-chemical laws of action, reaction and interaction and the similarity of the mechanical obstacles encountered by the different races of animals in similar habitats and environmental media.

The chief advance which has been made in the last fifty years is our abundant knowledge of the modes of adaptation as contrasted with the very limited knowledge yet attained as to the causes of adaptation. The law of adaptive radiation is a law expressing the modes of adapta- tion of form, which falls under the following great principles :

'1. Divergent adaptation, by which the members of the primitive stock tended to develop differences of form while radiating into a number of habitat zones.

2. Convergent adaptation, whereby animals from different habitat zones enter a similar habitat zone and acquire many superficial similarities of form.

3. Direct adaptation, for ezamplei in primary migration through an ascending series of habitat zones, aquatic or terrestrial, ar- boreal, aerial.

4. Beversed adaptation, where secondary migration takes the re- verse direction from aerial to arboreal, to terrestrial, to aquatic habitat zones.

5. Alternate adaptation, where the animal departs from an original habitat and primary phase of adaptation, and then returns from the secondary phase of adaptation to a more or less per- fect repetition of the primary phase by returning to a primary habitat zone.

6. Symbiotic adaptation, where vertebrate forms eihibit reciprocal, or interlocking adaptations with the evolution of other verte- brates or invertebrates.

��Law of

Adaptive Radiation

of Form

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