244 



CAUSES OF VARIATION 



Vital limits. Chlorophyllaceous plants are absolutely depend- 

 ent upon light for their very existence, but parasitic plants, 

 and animals in general, are not dependent upon light in any 

 vital sense, because they, like animals, subsist upon highly 

 organized materials in which the fixation of carbon has been 

 already accomplished by other organisms. 1 



All known facts indicate that animal life in general is essen- 

 tially successful in total darkness. Mules have been kept in 

 mines for twenty years, and beyond temporary sensitiveness of 

 the eyes no effect was perceptible. Prisoners have spent their 

 lives in dungeons. All embryonic development in mammals takes 

 place in the total darkness of the mother's body. It is doubtless 

 not too much to say that light has no effect whatever upon the 

 vital functions of the higher animals ; that it is as unessential 

 in this respect to animals as it is indispensable to plants. 



Bacteria, as a rule, not only do not need the light, and flourish 

 best in darkness, but strong sunlight is almost uniformly and 

 quickly fatal ; indeed, direct sunlight is recognized as one of 

 the most successful germicides (which is of itself the principal 

 reason why plenty of light should be provided wherever domestic 

 animals are kept). This fact is illustrated by inoculating a gela- 

 tin plate uniformly with bacteria, as Bacillus anthracis^ covering 

 the plate with a piece of black paper out of which some pattern 

 is cut, as the letter E, and exposing it all to strong sunlight for 

 a few hours. If the plate is then put into an incubator the bac- 

 teria will grow, except over the area exposed to the light, in 

 which area they have been killed. 2 From the well-known fact 

 that bacteria in a vacuum are not affected by light we conclude 

 that death is due to oxidation in the presence of light, a phe- 

 nomenon common in organized compounds. 



Light rigor. The movements of protoplasm in general are 

 retarded, or even stopped, in the presence of intense light, 

 so that rigor precedes death. There is therefore (for plants) 

 an optimum, minimum, and maximum intensity, and between 



1 This is written in general terms, and regardless of the fact that certain 

 lower organisms, whose nearest relatives are distinctly animal, themselves bear 

 chlorophyll. 



2 C. B. Davenport, Experimental Morphology, Part I, pp. 171-172. 



