FORM OF THE CONTINENTS— WATTS IQQ 



organisms alternating with periods of diversity. Again, at some 

 times connection seems to have been established north of the 

 Equator, at others to the south ; and we cannot ignore the possibility 

 of migration across polar lands or seas when terrestrial climates have 

 differed from the present. The facts of life distribution are far too 

 complex to be explained by any single period of connection followed 

 by a definite breaking apart, even if that took place by stages. Mrs. 

 Reid, too, has pointed out that resemblances between the Tertiary 

 floras of America and Europe actually increased at the time when the 

 Atlantic should have been widening. Unless continental drift has 

 been a more complicated process than anyone has yet conceived, it 

 seems impossible to escape from some form of the "land bridges" of 

 the older naturalists : 



Air-roads over islands lost — 

 Ages since 'neath Ocean lost — 



We have no right to expect greater simplicity in the life of a planet 

 than in that of an organism. 



As the question of drift must in the last appeal be one of fact, 

 it is not unnaturally expected that the real answer will come from 

 measurements of longitude and latitude with greater exactness and 

 over periods longer than has yet been possible. None of the measure- 

 ments hitherto made has indicated variations greater than the limits 

 of errors of observation. Two things, however, may militate against 

 a definite answer from this source. Many parts of the crust, such 

 as the shieldlike masses of Archean rock, may have completed their 

 movement, or be now moving so slowly that the movement could not 

 be measured. Careful selection of locality is essential, and at pres- 

 ent we have little guidance. Also, as the displacement of crust must 

 be dependent on the condition of its substratum, it will be a periodic 

 phenomenon and the rate of movement may vary much in time. 

 According to the theory of thermal cycles the subcrust is at present 

 solid and may not permit of drift. Drift, according to Joly and 

 Holmes, is a cyclical phenomenon; if present-day observations were 

 to give a negative result they would not necessarily disprove it. 



The occurrence of recumbent rock folds and nearly horizontal 

 slides or "nappes" in mountain regions gives positive proof that 

 parts of the upper earth crust have moved over the lower. In the 

 Northwest Highlands of Scotland a sliding of at least 10 miles was 

 proved by Peach and Home, and in Scandinavia it amounts to 60 

 miles. For mountain packing as a whole the figures already given 

 are far larger, while in Asia Argand has stated that packing of over 

 2,000 miles has occurred. Thus, when all is said and done, move- 

 ments on a colossal scale are established facts, and the question of 

 the future is how far we shall accept the scheme of drift due to 



