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Purpose.-The backround of all ecological work dealing with fluctuations in animal numbers and resulting population changes is laid in the analysis of environmental factors producing such fluctuations. Most of these factors are not simple, but rather are of a complex nature, representing the interaction of such variables as temperature, humidity, light, evaporation, etc. to produce greater or lesser fecundity, resistance to disease and to competition, and in some cases greater or lesser velocity of growth or development at a critical period in the life history, all of which influence population. Of the physico-chemical components of the environment, radiation within the range of wave lengths referred to ordinarily as light has received relatively little attention. The evidence which has been offered in many cases was gathered incidentally to other research. Most of the literature dealing with the problem of the effect of radiant energy of the visible spectrum upon rate of development or rate of growth was done previous to 1900 and suffers from the lack of adequately measured and controlled laboratory techniques. Recent work on radiation has been largely with the wave lengths above 700 my or below 320 mg. In the analysis of natural conditions the latter radiation plays a relatively small part. Beginning in 1934 and on the basis of preliminary work done in 1932 and 1933 on Physa gyrina the experiments covered in this paper were designed to determine: 1. The relationship of temperature to velocity of growth in Ilelisoma trivolvis pseudotrivolvis (F. C. Baker), considering only the period from fertilization to hatching of the egg; 2. What effect upon the temperaturevelocity curve, if any, could be observed when the embryos developed under the light passing through various filters; 3. The oxygen consumption of the eggs during the developmental period, and, 4. Some of the relationships between light, temperature and oxygen consumption. Literature.-Beginning with Edwards (1824) who claimed that tadpoles showed no development in the dark, and continuing through the work of Rugh ('35) who found no effect of wave length of light on development of frog tadpoles, the literature on the subject has contained almost every possible viewpoint on light-growth or light-development relationships. As Rugh and others have pointed out, probably most of the inconsistency is due to lack of control of other variables and the failure to distinguish between luminous intensity and radiation. Higginbottom (1850, 1863) found no differences in growth rates of tadpoles raised in light and dark. Beclard (1858) working with the larvae of Musca carinaria, reported that larvae grown in violet light, grew to be three times the size of those grown in green light. The sequence of colored lights, producing larvae in order of size from large to small, was violet, blue, red, yellow, and green. MacDonnell (1859) found no differences in the size of tadpoles raised in darkness and in light. Schnetzler (1874) reported a slight retardation of growth in darkness. He I Thesis submitted for the Doctorate at the University of Illinois, 1937.