328 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



Thus not only can F, consist of one of the parental types which continues to breed 

 true, or of several types, but the inheritance is changed by a change in external 

 conditions. He says : " Of considerable importance is the strong evidence which 

 points to the general conclusion that these " (inherited) " permanent variations arise 

 during the growth of the germ cells, and do not appear to arise before or after this 

 period " (p. 296). 



Tower's work may be said for the present to stand almost by itself In the 

 experiments of Sumner (84) on mice we have another example of inherited 

 temperature effect of a somewhat different kind. Sumner kept mice in a constant 

 but abnormally high temperature of 26° C, with the result that the ears, tails, and 

 feet grew noticeably larger and the coat more hairy than in the control animals kept 

 at ordinary temperatures. These characters were found to be transmissible to an 

 appreciable extent through a number of generations, when the offspring were returned 

 to normal temperatures. 



The experiments of Standfuss, Fischer, and other workers, of exposing the pupae 

 of butterflies and moths to abnormal temperatures, are of a somewhat similar nature. 

 They indicate that moderate degrees of heat and cold tend to alter these insects so 

 that they frequently assume to a certain degree the characters of distinct varieties 

 of these species found in colder and warmer climates. Thus heat and cold, in this 

 case, seem to upset the normal stability of the species type, and the germ cells are 

 altered, although not necessarily in the same way as the somatic cells. 



In Simocephalus vetulus, Agar(1) has produced a difference in the body length by 

 keeping it for some generations under an abnormally high temperature. This is shown 

 by a diminished size accompanied by a reduction of the number of young per brood. 

 These changes are retained for several generations when the animals are returned to 

 normal conditions, but are gradually lost, and the animal Anally assumes the normal 

 type agtiin. Woltereck (100) has obtained somewhat similar results with Daphnia. 



Kammerer (45) has produced colour changes in Lacerta by exposing them to 

 abnormal temperatures ; in this manner he produced female dimorphism in one 

 species and male dimorphism in another. These induced conditions behaved in 

 a Mendelian manner in crosses. Their behaviour, however, was not adequately 

 established by carrying them through a sufficient number of generations. 



When we turn to the case of plants, we find many striking instances of the action 

 of temperature on inheritance which apparently persists through many generations. 

 Many low plants, such as the bacteria and yeasts, have been made the subjects 

 of extensive investigations of this kind. Their rapidity of growth and multiplication 

 and the ease with which experiments on them can be controlled make them favour- 

 able material for work of this nature. It has been shown by Pringsheim (74) that 

 many of the adaptations shown by micro-organisms to abnormal temperature 

 conditions become fixed, and are apparently transmissible through many generations 

 both sexually and by means of spores. 



