MATTHEW, CLIMATE AND EVOLUTION 305 



A fourth instance cited by Dr. Scharff is the distribution of the river- 

 mussel Margaritana, and as he well observes, numerous other instances 

 would probably show similar discontinuous distribution. But, so far a^ 

 I have been able to find such instances, the same reasoning and the same 

 explanation apply to them all. 



Ceiticism of Some Opposing Hypotheses 



It is not practicable to take account here of the flood of paleogeo- 

 graphic discussions of recent years which have advocated all sorts of 

 consistent or inconsistent changes in continental outlines. They agree 

 for the most part in failing to take into account certain considerations 

 which to my mind are essential elements in any problem of distribution. 



Among the geological considerations are the following : 



1) E\adence that the present distribution of the deep ocean basins is 

 in the main due to isostatic balance. This affords a strong presumption 

 in favor of its permanence. 



2) Absence of abysmal deposits in the geological formations of any 

 continental region. Chalk deposits are not an exception, as it has been 

 shown that they were deposited in shallow epicontinental seas rather than 

 in deep oceanic basins. 



3) Abrupt ending of an elevated line of disturbance and its continua- 

 tion as a submerged line of disturbance does not necessarily indicate that 

 the submerged portion was formerly elevated, although it does reduce 

 the improbability of its former elevation by indicating a line of dis- 

 turbance and hence of possible elevation. 



4) The presence of marine formations of Cretaceous or Tertiary age 

 over large portions of the interior of the great continents does not indi- 

 cate that these continents first came into existence as such during the 

 Cretaceous or Tertiary. In the better known portions of the earth's 

 surface we know well enough that these marine formations were due to 

 periodic temporary submergence, interrupted by periods of more or less 

 complete emergence. It is but reasonable to apply the same explanation 

 to the less known regions. I see no more reason to suppose, as do Von 

 Ihering, Scharff and others, that South America first came into existence 

 as a united continent in the Tertiary, than to conclude on similar evi- 

 dence that North America was but a group of isolated land masses until 

 the end of the Cretaceous. In this countr}^, we have positive proof of 

 its antiquity; but the evidence for recent origin of the South American 

 would apply just as well to the North American continent. A similar 

 presumption of antiquity applies to Australia, Asia and Africa. 



