The central interpretation advanced in this paper 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. Conditions which accelerate the developmental rate accelerate and accentuate the differentiating tendencies as well as the growth tendencies. Retarding conditions operate in the opposite manner. This relation of growth and differentiation is evident in the course of normal development. Accelerating conditions hasten growth in the early stages, but later bring about a slackening of the growth rate, probably because of their accentuated action on growth inhibitions. Retarding conditions cause a protraction of the early growth, but a less abrupt slackening of the growth rate with age, so that eventually the size attained is often greater than under accelerating conditions. Similarly, the addition of somites, which is practically synonymous with the growth of the embryo, proceeds faster under accelerating than under retarding conditions of development, but terminates relatively sooner and more abruptly. As a consequence, warm or brackish water forms of a species of fish have as a general rule fewer vertebrae than the forms inhabiting cooler or more saline water. An increase in the number of segments is also associated with protracted development having' a basis in a great increase in the amount of yolk in the egg or in the larval-postlarval modification to pelagic life in which the muscle is replaced by gelatinous tissue. Fishes passing through a protracted development often show an extenuation of age differentiation in form as well as of growth. Their proportions depart more from the juvenile conditions than do those of fishes going through a more hurried development. They often show also an increase in dermal modifications, as cirri, plates and spines. This increase in differentiation is more than proportionate to the extenuation of growth; at comparable sizes, the fishes passing through the contrasting types of development show these differences. The action of differentiation controls (inhibitions) is obviously hastened and accentuated under acceleration conditions. Some supposedly contradictory cases are shown to be based on erroneous observations. Others, however, are valid, but can not be explained at present. The problem of adaptation.-The modifications of developmental rate entailing definitive structural consequences seem best interpretable as physiological adaptations. When genetic, the changes in developing rate may be regarded as due to the selection of metabolic mutations, or to egg or larval modifications. Many of these structural consequences of physiological adaptations are among those characters of high systematic value long thought to be specifically devoid of adaptational significance. The role of adaptation in evolution is probably more extensive than has generally been postulated. The problem of speciation.-The structural consequences of differential rate of development probably comprise a considerable proportion of the characters differentiating local races of fishes. The problem of degegeneration.-Degeneration resulting from accelerated development consists merely in the retention of juvenile characters. Degeneration correlated with decreased physiological activity and retarded development may be much more extensive, involving the elimination of final stages of early development and the retention of embryonic as contrasted with juvenile characters. The problem of progressive evolution.-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 problems of phylogeny.-The interpretation of degeneration being associated with protracted development offers a means of testing whether simplicity of structure is a primitive or secondary feature. The general interpretations of the paper indicate that structural evolution controlled by alterations in developmental metabolism is approximately or even exactly reversible. Such changes involve particularly modifications of the adult toward or away from juvenile characteristics. If such changes are reversible, as seems highly probable, then the biogenetic law does not apply.