NEW TYPES AMONG ANIMALS 157 



and night averages about 60** with a noon temperature of 65** to 

 70°, and (5) the work of students with a maximum when the tem- 

 perature averages about 40°. We have seen in previous pages 

 that a similar curve can be drawn for health with a maximum at 

 an average temperature of 64 °F. Thus it appears that for each 

 species there is a certain temperature at which the various func- 

 tions take place most nearly in the normal fashion. Above or 

 below that temperature there are departures from the normal, 

 and among insects these departures take the form of new varieties 

 or mutants. Likewise, as appears from Tower's work and from 

 our study of the human deathrate, there is an optimum humidity, 

 above or below which there may be similar changes although this 

 is not so well demonstrated. 



Let us now turn from insects to some of the higher animals and 

 see what effect changes of environment have upon their offspring. 

 Kammerer has caused what seem to be hereditary changes by 

 subjecting the nurse-toad (Alytes obstetricans) to a high tem- 

 perature. This animal is a small toadlike amphibian about two 

 inches long, with a gray color, plump form and warty skin. It 

 is remarkable as the only European batrachian in which the male 

 helps in rearing the offspring. Hence the name. The eggs are 

 larger than those of related animals. They are also fewer and 

 have more yolk. At the time of spawning they are stuck together 

 in two strings which the male twists around his legs. He then 

 returns to his usual haunts among stones and sticks. The nurse- 

 toad is thoroughly terrestrial, but prefers moist places, and feeds 

 at night. On very dry nights it may enter the water in order to 

 keep the eggs sufficiently moist. Three weeks after the eggs have 

 been laid the male regularly enters the water and stays there 

 until all the eggs are hatched and the young tadpoles are swim- 

 ming freely about. 



Kammerer kept the toads at a temperature of 77° to 86°F, 

 a temperature which is rare in the cool, shady places frequented 

 by the animals in central Europe. This led them to seek the 



