Page:Popular Science Monthly Volume 76.djvu/329

poles of three weeks of age to withstand an excessive amount of salt than it is younger larvae.

But it is not my contention that younger stages of ontogeny may not be more sensitive than older ones to certain abnormal external conditions. A young chick is more sensitive to cold than an older one. My contention is that if we wish to produce a modification, which is nevertheless compatible with life, we can succeed best with the younger stages of development and with lower organisms. In this case no life-compatible modification was produced in either case. The younger tadpoles modified, but died. The older ones did not modify, but continued their development in a normal way. The real reason of the difference may here have been that the vital cells of the older tadpoles were better protected by the outer covering (ectoderm), so that the interchange of fluids (osmosis) worked more gradually in these vital cells. In just the same way the cold injures the young chick more, because from the absence of feathers, the same degree of cold reaches his vital organs more suddenly.

Experiments involving the effects of electricity on the growth of frogs' eggs are not of any special significance. They produce changes in the arrangement of the pigment and sometimes abnormal cleavage, or abnormal development. Differences of atmospheric pressure (presumably really differences in the amount of oxygen absorbed by the water) cause differences in the rate of growth. The researches of Rauber^[1] show that

At a pressure of three atmospheres no growth occurred. At a pressure of two atmospheres growth was slower than at the normal pressure. At three fourths of an atmosphere death generally occurred. Thus the optimum condition of oxygen tension is near the normal of the atmospheric.

This certainly can not be considered a surprising discovery, nor have the experimenters produced any appreciable modification on amphibians by means of differences in the amount of oxygen.

The development of tadpoles can be considerably retarded by scanty feeding, so that they may be kept in the gill-breathing stage for over a year; but if they survive they still retain their potentialities for becoming normal adults. This is shown by an interesting experiment of de Varigny's. He describes it thus:

I have myself kept toads in the tadpole state for over two years, merely by feeding them very scantily. They were born in the spring of 1889, and remained all the time in an aquarium in the laboratory, having water enough at their disposal, being always sufficiently provided with aquatic plants, and enjoying heat enough; but it can by no means be said that their evolution was arrested by the cold of winter, as often happens in mountain ponds, when the cold of autumn sets in before the tadpoles have achieved their development, so that they become frogs or toads only in the course of the following year. In the

1. Davenport, "Exp. Morph.," p. 306.