THE FACTORS OF EVOLUTION 193 



inhabitants of dark caverns are disposed to lose the pigment 

 of their body surface. The famous denizen of the Adelsberg 

 grotto, Proteus, is quite colourless. But if it is kept for some 

 time in a well-lighted aquarium the skin begins to form pigment 

 and its colour becomes darker. 



Light may further have considerable influence upon the 

 course of the processes of regeneration. On this subject Loeb, 

 whose valuable labours have elucidated many difficult points, has 

 made a series of remarkable experiments. In every marine 

 aquarium is found a pretty polyp-stock, Eudendrium ramosum, 

 a relative of our fresh-water polyp Cordylophora lacustris. 

 These organisms are highly sensitive, and after the first few 

 days of their captivity, in consequence of having been handled, 

 throw off all ' heads,' only the bare trunk remaining. If the 

 aquarium has plenty of daylight the damage is quickly repaired, 

 for in a few days the trunk has regenerated numerous polyps. In 

 the dark, however, not a single head appears even after many 

 weeks. A similar result is obtained if we expose them to 

 different-coloured light ; blue rays favour regeneration, while 

 red rays have the same effect as complete darkness. Finally, 

 if we expose to red light the polyps that were formed in blue 

 light they inevitably perish. 



Plants are to a still higher degree affected by light. Light is 

 not only an essential condition of the nutrition of all green plants, 

 but controls also in a high measure their growth, formation of 

 organs, and generally the whole external appearance. What 

 an enormous difference is there for instance between the Black 

 and the Lombardy Poplar. The cause of this dissimilarity is 

 the light. In the Black Poplar generally the upper side of 

 the branches is exposed to the daylight, and it is only here 

 that the leaf-buds are able to develop. Exactly the reverse is the 

 case with the high-rising branches of the Lombardy Poplar, 

 which are mainly illuminated on the under side and therefore 

 develop the leaves below. 



Most striking is the correlation of form and the effects of 

 light in the organs of plants which carry the chlorophyll and 

 carry on assimilation. In the normal state the stems of the 

 Prickly Pear (Opuntia), which is very common in the countries 

 of the Mediterranean, are greatly flattened and therefore often 

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