TRANSMISSION OF MODIFICATIONS 379 



All things considered, the conviction is forced upon us that 

 the Shetlands suffered a progressive diminution because of low 

 temperatures or short feed, or both, or else that these northern 

 forms, living under hard conditions, lagged behind their more 

 fortunate neighbors in the general increase in size that has 

 attended the evolution of the horse kind generally. 



The temperature of hot springs varies all the way from 50 to 

 98 C. 1 So far as known they are all inhabited by living organ- 

 isms. The protoplasm of ordinary plants and animals cannot en- 

 dure a temperature above 45. Death quickly follows the attempt 

 to raise it much above this point, and the nearest relatives of 

 these hot-springs species are no exception to the general rule. 



Yet the fact remains that the hot springs are inhabited, and 

 by creatures so small that their temperature must be the same 

 as that of the waters in which they live. How have these waters 

 become peopled with organisms living in temperatures ten to 

 fifty degrees higher than could be endured by the stock from 

 which they must have descended ? 2 



No amount of selection could account for the fact, for there 

 are no other known species living in an environment approach- 

 ing these temperatures. There must have been progressive de- 

 velopment of some fashion and from some cause . 



Upon this point the experiment of Dallinger is both signifi- 

 cant and valuable. 3 He reared Flagellata in an oven where 

 control of temperature was absolute. Beginning at 15.6 C., 

 he took four months in which to raise the temperature through 

 5.5, a precaution now known to be unnecessary, as Flagellata 

 will endure a quick rise to 21. 



When the temperature reached 23 the organisms " began 

 dying, but soon ceased, and after two months the temperature 

 was raised half a degree more." After a time it reached 25.5, 

 when they again began to die, and for eight months the temper- 

 ature could not be raised even a quarter of a degree above this 



1 C. B. Davenport, Experimental Morphology, Part I, pp. 250-251. 



2 The temperature varies slightly in different parts of hot springs, being lower 

 near the edge. 



3 C. B. Davenport, Experimental Morphology, Part I, pp. 252-254; Vernon, 

 Variation in Animals and Plants, pp. 379-380; Journal of the Royal Microscopical 

 Society, VII, 191. 



