ORGANIC EVOLUTION — DAVENPORT 423 



I have dwelt at length on the Collcmbohi of the hcacli because they 

 may serve to ilhistrate the principle that mutations become the char- 

 acters of sjiecies and play a part in evolution provided they meet 

 some demand of the environment; or, the other way round, a new 

 mutation persists as a species character if it can find an environment 

 to which it is suited. 



This general principle is of wide application. In Banta's 

 Daphnids there appeared a female whose young died on a cool day 

 in the autumn. It was found that subsequent broods could be kept 

 alive in an incubator at a higher temperature than that of the 

 room. In short, an investigation of the temperature relations of 

 these cold-sensitive 3'onng and their equally cold-sensitive descend- 

 ants showed that there had arisen by mutation a thermal clone, a 

 parthenogenetically reproducing strain, whose optimum temperature 

 was about 10° C higher than tliat of the ordinary Daphnids. This 

 mutation was fatal at the or<linary room temperature; it had im- 

 portant survival value for the environment of an incubator; it would 

 have had an important evolutionary value had there been a warm 

 spring near by into which the strain could have been transplanted. 

 This experience, indeed, shows the probable method by which 

 aquatic animals have come to inhabit hot springs. It is not by 

 gradual change wrought on the germ plasm by the direct action of 

 the high temperature of the water, but rather the fine opportunity 

 for survival afforded by the high temperature to any chance 

 thermal mutant. 



Again, as has long been known, many of the animals that live in 

 caves are blind and much speculation has been offered to account for 

 this blindness. The old idea was that, through disuse and the parsi- 

 mony of nature that would prevent it from continuing to form useless 

 organs, the useless organs were no longer formed. On the other hand, 

 Eigenmann, through his extensive knowledge of fishes, was able to 

 point out that the blind fish of caves belonged to just one family of 

 fishes, a family that had mutated in the direction of blindness in 

 various parts of the continent. Now, some of these mutations in the 

 direction of blindness have survived even where there are no caves, 

 but where there are waters running through densely wooded swamps 

 and characterized by dark holes where poor sight is no handicap to 

 the fish. When a blind mutation arose in that family of fishes living 

 in the region of limestone caves of Indiana and Kentucky, that muta- 

 tion was no handicap to its possessor. For the possessor had other 

 sense organs sufficient to secure its prey. The waters of the cave, 

 indeed, removed competition, and in other ways afforded an extra- 

 ordinarily favorable environment for this genus of fish. 



Another illustration may be afforded by still another group of ani- 

 mals. As you know there are vast numbers of mollusks living in the 



