COLOEATIOK AFFECTED BY THE ENVIEONMENT. 65 



mental cause— probably light— was responsible ; there is 

 obviously no room here for natural selection. 



Absence of Colour in Cave Animals. 



In certain parts of the world, notably in Kentucky and in 

 Carolina, there are immense caves excavated by the agency of 

 water in limestone rock, and possessing a peculiar fauna and 

 flora of their own. The Mammoth Cave, in Kentucky, which 

 is one of the best known, consists of a complicated network of 

 ramifying passages, the total length of which is no less than 

 150 miles. Some parts of the cave are drier than others, as is 

 shown by the absence of stalagmitic deposits. Streams course 

 through some of the passages, and there are deep pools here 

 and there ; there is thus provision for both aquatic and terres- 

 trial animals and plants. These caves are, of course, quite 

 dark, and their temperature is remarkably uniform. Dr. 

 Packard * made a series of observations upon the temperature 

 of the Mammoth Cave, and found that it never rose above 

 56° Fahr., or fell below 52° 5'Fahr.; the mean temperature for 

 the summer being 54° Fahr., that for the winter 53° Fahr. 



These conclusions were confirmed by another observer ; and 

 it appears, therefore, that the variations of temperature through- 

 out the year are very small, and, small as they are, are no doubt 

 gradual. It would be expected that such remarkable physical 

 conditions, so different from those which obtain outside the 

 caverns, would produce some effect upon the animals and plants 

 permanently resident in those caves. There is a certain 

 analogy between life in these caves and life at the bottom of the 

 ocean ; the chief resemblance, of course, lies in the darkness of 

 both situations. As to temperature, there is a uniformity in 



' See, for a detailed account of the North American caves and a general 

 summary of the facts known about the subject, Br. Packard's " The Cave 

 Fauna of North America," etc., .1/em. Nat. Acad. Sci., vol. iv., 1889. 



5 



