DIMINISHING ENVIRONMENTAL INFLUENCE 3*5 



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 con- 

 ditions. 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 con- 

 tinued 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, be- 

 cause 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 (pre- 

 sumably really differences in the amount of oxygen absorbed by the 

 water) cause differences in the rate of growth. The researches of 

 Eauber 28 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 condi- 

 tion 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 am- 

 phibians 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 becom- 

 ing 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 



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



