312 CAUSES OF VARIATION 



The same author mentions snails living in France at 35 to 

 36C., and in Padua at 50; rotifers at Karlsbad at 45 to 54; 

 frogs at " Pise " at 38, and the crustacean Cypris balnearia at 

 Hammam-Meckoutin at 81. 



All these are living at temperatures much above the death 

 point of their nearest relatives, and we are forced to the conclu- 

 sion that they have descended from ancestors living in tempera- 

 tures not above 40 and not at all able to endure the extreme 

 temperatures in which the present generations live and thrive. 

 Acclimatization there must have been, in some way and at some 

 time, and that of extreme degree, involving profound changes in* 

 the protoplasm. 



Experiments throw interesting light upon the manner of accli- 

 matization. " Dutrochet found that if the plant Nitella was 

 put into water at 27, the currents in the protoplasm were 

 stopped, but soon began again. If put now into water at 34, 

 they again stopped moving, but in a quarter of an hour began 

 once more. If then put into water at 40, the currents slowed 

 down, but began again later." * 



Davenport and Castle reared two lots of tadpoles from the 

 same lot of recently laid eggs. One lot was kept at 24 to 

 25C., the other at 15. " Both lots developed normally, the 

 former much more rapidly." At the end of four weeks both 

 lots were tested for heat rigor by gradually heating the water 

 in which they were contained. Those reared at 15 became 

 motionless at 41 or below ; while of those reared at 24 to 25 

 (10 higher), no tadpole died under 43, the average increased 

 resistance being 3-2. 2 The experimenters remark that this in- 

 creased resistance was due in no sense to selection, for no 

 deaths occurred during the period of acclimatization. It must 

 have been due to changes wrought in the protoplasm. This con- 

 dition was more or less lasting, for the difference in -resisting 

 power, though lessened, was still noted after seventeen days' 

 sojourn in cooler water. 3 



1 Morgan, Evolution and Adaptation, p. 320 ; C. B. Davenport, Experimental 

 Morphology, Part I, p. 252 (tables). 



2 C. B. Davenport, Experimental Morphology, Part I, p. 253. 



3 Ibid. p. 254. 



