356 O^^ THE ANTIQUITY OF TIIP: CAVERNS 



organic life be entirely the product of certain internal generating forces, still more, if it l)e 

 the result of anything like special creative acts, then it is impossible to find any good 

 reason for this correspondence ; if, on the other hand, it is largely the result of the in- 

 fluences brought to bear npou it by the external world, then we should expect just such 

 changes as we find here. 



If we accept these changes as the result of environment, complicated it may be by other 

 conditions, but still the result of environment, then it becomes a matter of the first hnpor- 

 tance to determine the length of time during which these changes have been going on. 

 This, if it can be done, will serve to give us a measure of the rate at which organic life can 

 accommodate itself to new conditions. Not that this measure will have a high degree of 

 certainty, or will be applical^le to all organic forms ; we must, however, value anything 

 which approximates a determination of the rate of change which a modification of condi- 

 tions can bring aljout. 



o 



I. The Coxditioxs of the Formatiox of Cavekxs. 



In order to understand the limitation of cavern life, it is necessary for us to see clearly 

 the conditions which make this life possible, and the horizontal and vertical extension of 

 these conditions. 



The first point is to determine what makes caverns possible. Only a very small part of 

 the earth's surface presents conditions favorable to the existence of caverns, so we are 

 justified in the conclusion that the conditions determining their formation are definite 

 and limited. More than this, we find in manj^ countries caverns which have become closed, 

 or are rapidly closing, so that we may reasonably conclude that the conditions which favor 

 their formation may exist at one time and pass away at another. 



We will leave aside the caverns sometimes formed in cooling lava streams hy the fluid 

 interior of the mass sinking below the solidified crust. There is but a smaU amount of 

 cavern region where this class of facts is possible ; moreover, such caverns are essentially 

 superficial, and are rapidly destroj'ed as the friable lava wears away. Besides, these cav- 

 erns are generally limited to regions which are desolate as long as the caverns continue 

 to exist, and they therefore promise us little in the way of experiments on the variability 

 of organic forms. 



Tlie ordinary t3'pe of caverns, and that to which we are to give our attention, are the 

 product of the action by water, aided, it may be, by sundry other minor causes. There 

 are two distinct divisions of water worn caverns, those formed by the sea, and those foi'med 

 by fresh water in the form of subterranean streams. Caverns of the first named class, 

 though common enough, are for our investigation quite unimportant. Sea waters having 

 little solvent power, the caves they form are generally quite shallow, rarely attaining great 

 depth, and generally opening broadly to day. It is very rarely the case that marine caves 

 have the depth necessary to enable them long to survive after the sea has ceased its exca- 

 vation. Along the whole Atlantic coast, from Nova Scotia to Mexico, I do not know of a 

 single cavern deep enough to give darkness, and above the present level of the sea. The 

 only marine caves on the American coast of any considerable depth personally known to 

 me, are on the Magdalen Islands m the Gulf of St. Lawrence, but 1 do not know at that 



