810 On Certain Factors of Evolution. 
nate, even if the plants do not bear fruit, can only be explained by 
the absence of light ; and perhaps this is an important cause of the 
absence of all plant life in the ocean below a depth of about 300 to 
500 fathoms, Certainly there are ample means for the colonization 
of caves by vegetables; the temperature, moisture, and inorganic 
food are more favorable than the sum total of conditions on alpine 
summits or in the high polar regions, or in hot springs. 
Animal life can apparently withstand greater physical obstacles 
than vegetable. As regards the struggle for existence, it possibly 
exists to a limited extent in cave animals. There is probably not 
enough vegetable or decayed animal food for all the animals, and 
some may die of hunger. The carnivorous beetles and Arachnida 
perhaps have a less favorable chance to obtain living food than the 
Crustacea, for the blind crayfish have a tolerable abundance of food 
in the Cæcidotæa, perhaps the most abundant form found in caves 
containing underground waters. 
We may, with Darwin, for convenience, use the phrase “ natural 
selection ” to express the process by which Whe cave fauna was pro- 
duced, but such a term to our mind expresses rather the result of 
a series of causes than a vera causa in itself. There is of course 
no doubt but that many animals carried by different means into 
caves cannot thrive there, and consequently die. It is only those 
which have been able, by certain peculiarities of their life in the 
upper world allied to cave existence, to adapt themselves to cave 
conditions which permanently breed there. Such forms, it is con- 
venient to say, have been by nature selected and are successful in 
colonizing the darkest and most forbidding and apparently hopeless 
corners in the earth’s crust. But such a phrase as “ natural selec- 
tion,” we repeat, does not to our mind definitely bring before us 
the actual working causes of the evolution of these cave organisms, 
and no one cause can apparently account for such a result. There 
is rather a complex assemblage of physical causes, all working 
together, to secure a harmonious result. The most important and 
potent of these causes, when we study them under such appreciable, 
because so extraordinary, conditions as the physical features of cave 
existence, would seem to be the following : os 
1. Change in environment from light, even partial, to twilight 
or total darkness, and involving diminution of food, and compensa- 
tion for the loss of certain organs by the hypertrophy of others. 
