SCHMIDT: ANIMAL GEOGRAPHY 775 



most authority on fossil insects, Handlirsch analyzed the existing faunas of the 

 continents and showed that even for this ancient and eminently terrestrial group 

 the proposed connections of continents in the southern hemisphere are flatly 

 opposed by a vast mass of contrary evidence. Only an uncritical enthusiasm 

 could maintain them on faunal grounds alone; as we shall now see, there is 

 crucial geological evidence against them. 



ISOSTASY 



The whole scheme of rising and sinking continents is now found to be opposed 

 by unshakable geological evidence, from the facts summarized as "isostasy," 

 which show that the continental platforms are indeed stable. The whole mass of 

 books, journal articles, and addresses to scientific meetings on the subject of land 

 connections is an incredibly futile chapter in the history of animal geography. 



When the survey of the earth's surface had advanced to large-scale operations 

 it was discovered that astronomical determinations of latitude were sharply at 

 variance with direct measurement. The north-south breadth of Puerto Rico as 

 found by astronomical calculation, for example, differs by about a mile from the 

 50-mile direct measurement. This difference results from a deflection of the plumb 

 line toward mountain masses and toward continental masses. J. H. Pratt reported 

 in 1855 "On the Attraction of the Himalaya Mountains and of the Elevated 

 Regions Beyond Them Upon the Plumb Line in India." Since that time a gravity 

 survey of the world has been made by means of the pendulum observations begun 

 by George Biddell Airy at about the same time. The result of the world survey, 

 which constitutes the science of geodesy, has been to re-enforce the validity of 

 the principle of isostasy. It is found that the granitic materials of the mountains, 

 the "sial" of Suess, have a density distinctly less than that of the basaltic rocks 

 underlying the oceans, the "sima," in the proportion of 2.7 to 3.0. The sial, in a 

 sense, floats on the heavier substratum of sima, and it is the sima that forms 

 the ocean bottoms. The continental bases extend deep into the sima, with further 

 great downward extending masses beneath the mountain ranges, except where 

 their isostatic adjustment is not complete, as shown by the occurrence of earth- 

 quakes. An example of continuing isostatic adjustment familiar to us in North 

 America is the rise of the northeastern quarter of the continent after the retreat 

 of the continental glacier; the rare earthquakes in this otherwise extremely stable 

 area are ascribable to the readjustment of the continental block with the disap- 

 pearance of the ice load. The conviction that the continental platforms are indeed 

 permanent in broad outline, and that the ocean floor is of very different compo- 

 sition, became more and more an axiom of modern geology as the extension of 

 gravity measurements failed to find exceptions to the lightness of the continental 

 masses relative to the ocean floors. This flatly contradicts the hypotheses of vast 

 former continents extending across the existing oceans. 



Continental Drift 



Just when this began to be realized, an ingenious alternative was afforded by 

 the hypothesis of continental drift elaborated by Alfred Wegener (1915), from 

 ideas already current (Taylor, Baker). ^ Wegener, impressed by the current 



1. Du Toit (1937) sketches the history of the ideas involved. 



