The Structural Consequences of Modifications of the Developmental Rate in Fishes, Considered in Reference to Certain Problems of Evolution

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Abstract
The central interpretation advanced is that either individual or racial modifications of the developmental rate affect the differentiation aspect of development in the same direction as the growth aspect. Accelerating conditions accentuate the differentiating tendencies as well as growth, but bring about a more abrupt slackening or cessation of development, which in the end is carried farther under retarding conditions. Under retarding conditions the addition of somites is slackened but carried farther, to produce individuals or forms with an increased number of vertebrae; and age differentiations in form are also carried farther. The changes in developmental rate may be regarded as metabolic mutations of adaptational significance: the role of adaptation in evolution is probably more extensive than usually supposed. The structural consequences of differential rate of development probably comprise a considerable proportion of the characters differentiating local races of fishes. These structural consequences may involve degeneration. Accelerated development results in the emphasis of early differentiations, and seems to provide a means for the fixation and perpetuation of variations leading toward specialization. The interpretations advanced offer a means of testing whether simplicity of structure is primitive or secondary. Structural evolution controlled by alterations in developmental metabolism is often approximately or even exactly reversible. If such changes are reversible, the biological law does not apply to them. A pertinent bibliography is appended.

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