3 2 4 THE POPULAR SCIENCE MONTHLY 



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 

 0. Hertwig and by Lillie and Knowlton. Cold retards the rate of 

 growth considerably; but the point here is, that if these animals con- 

 tinue to develop at all, the adult forms are not essentially different 

 from the normal. Permanent differences in size, such as are pro- 

 duced in snails and especially in plants, are not effected among the 

 amphibians by differences in amount of heat. Extra high tempera- 

 tures, 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 tempera- 

 ture 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. 25 Grav- 

 ity, on the other hand, has considerable demonstrable influence on cleav- 

 age during the very early stages of the developing egg. 26 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 in- 

 fluence in the later stages is certainly very slight and, so far as we can 

 see, negligible. My argument is that the modifying influence of grav- 

 ity 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. Eetardation 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 generali- 

 zation which I am making. H. de Varigny who has himself experi- 

 mented upon the effect of introducing common salt into fresh water 

 where tadpoles are growing, states 27 that it is easier to accustom tad- 



25 Morgan, " Exp. Zool.," pp. 262-263. Vernon, p. 249. 



26 Morgan, "Exp. Zool.," p. 267. 



27 " Exp. Evolution," pp. 189-190. 



