sure and food. Experiments with differences of temperature cause merely differences in the rate of growth. These were first performed by Higginbottom some sixty years ago and have been repeated by O. Hertwig and by Lillie and Knowlton. Cold retards the rate of growth considerably; but the point here is, that if these animals continue to develop at all, the adult forms are not essentially different from the normal. Permanent differences in size, such as are produced in snails and especially in plants, are not effected among the amphibians by differences in amount of heat. Extra high temperatures, or those above the optimum, will often produce abnormalities or monstrosities, such as embryos with double heads or tails, but these do not live.
It is also noteworthy that the latitude for possible manipulation, by the use of high temperatures, is not as great here as among plants. Vernon says, page 228:
Better instances, of the more and more unfavorable influence of increasing high temperature, are found among plants: as in them the optimum temperature is much further removed from the "maximum" temperature (the highest temperature at which growth can take place at all) than it is in animals.
Light has but little effect upon the growth of amphibians.[1] Gravity, on the other hand, has considerable demonstrable influence on cleavage during the very early stages of the developing egg.[2] This influence of gravity on the growth of amphibians should be contrasted with its far greater effect on plants and hydroids, and also with its probable effect on the developing embryos of mammals, which must here be very slight, if any, judging from the haphazard nature of placental attachments. Gravity may of course play a certain role in mammalian embryology in the very youngest stages. Investigation of this question would in the nature of things be very difficult. But even if it does, its influence in the later stages is certainly very slight and, so far as we can see, negligible. My argument is that the modifying influence of gravity is less in higher organisms than it is in lower, and less in older stages of development than it is in younger.
The effect of changes in salinity and density of the water in which frogs are developing are in general similar to those involving changes in the temperature. Retardation of the rate of growth may be brought about, but the evidence is lacking to show that the animals are any different in the end. It is of interest to note that in this connection there is something that might be thought contradictory to the generalization which I am making. H. de Varigny who has himself experimented upon the effect of introducing common salt into fresh water where tadpoles are growing, states[3] that it is easier to accustom tad-
