this is true we should not expect any changes in the other organs of the body, since these other organs have not experienced any change from their normal environment. If, on the other hand, the environment is actually and certainly changed, as is the case when caterpillars are fed on different kinds of food, the variations associated with sex may, to a certain extent, be artificially induced.[1] It is very doubtful if the vertebrates will permit any such modification of sexual differences by changes in their diet.
Fishes
One of the first points noticed in looking over the results of modification experiments on fishes is that the total number of such experiments is significantly small. There is little to record that can in any way be considered striking or interesting as bearing on the present discussion.
If the under surface of flounders is exposed to light during the growth of the fish the side which is normally white will become pigmented like the upper side, though not so much so.[2] Here we see that what is perhaps the most striking modification produced in a fish is concerned merely in a change of pigmentation, which is probably always one of the easiest changes to bring about.
In hatcheries, it is sometimes desirable to retard the growth of fishes for commercial reasons. The experiments of Meyer, Earll, Rice and others show that lowering the temperature of the water lengthens the interval between fertilization and hatching to about a month. There are no further facts to show whether the fishes are permanently modified thereby. Presumably they are normal in the end, as is the case with retarded insects and frogs. Fishes show good regenerative powers and in accordance with their phylogenetic rank.
Fishes and frogs will not endure the high atmospheric pressure experiments that can be brought to bear upon low invertebrates without loss of life. Although the lower forms which submit to these high pressures (200 to 600 atmospheres) are only temporarily modified and afterwards regain their normal proportions,[3] the facts are significant as showing that higher protoplasm will not submit to the rude and abnormal treatment that the lower will. The higher protoplasm must have its accustomed environment and will not survive if it is ruthlessly forced into very unnatural surroundings.
Amphibians
Within this group one finds modifications brought about by differences in temperature, light, gravity, salt, electricity, atmospheric pres-
