262 THE GEOLOGICAL HISTORY OF PLANTS. 



two successive periods have existed in the same region, 

 and under circumstances that render it probable that 

 plants have continued to grow on the same or adjoining 

 areas throughout these periods, the comparison becomes 

 direct, and this is the case with the Erian and Carbonifer- 

 ous floras in northeastern America. But, when the 

 areas of the two formations are widely separated in space 

 as well as in time, any resemblances of facies that we may 

 observe may have no connection whatever with an un- 

 broken continuity of specific types. 



I desire, however, under this head, to affirm my con- 

 viction that, with reference to the Erian and Carbonifer- 

 ous floras of North America and of Europe, the doctrine 

 of "homotaxis," as distinct from actual contemporaneity, 

 has no place. The succession of formations in the Palaeo- 

 zoic period evidences a similar series of physical phenom- 

 ena on the grandest scale throughout the northern hemi- 

 sphere. The succession of marine animals implies the 

 continuity of the sea-bottoms on which they lived. The 

 headquarters of the Erian flora in America and Europe 

 must have been in connected or adjoining areas in the 

 North Atlantic. The similarity of the Carboniferous flora 

 on the two sides of the Atlantic, and the great number of 

 identical species, proves a still closer connection in that 

 period. These coincidences are too extensive and too fre- 

 quently repeated to be the result of any accident of similar 

 sequence at different times, and this more especially as 

 they extend to the more minute differences in the feat- 

 ures of each period, as, for instance, the floras of the 

 Lower and Upper Devonian, and of the Lower, Middle, 

 and Upper Carboniferous. 



8. Another geographical question is that which relates 

 to centres of dispersion. In times of slow subsidence of 

 extensive areas, the plants inhabiting such areas must be 

 narrowed in their range and often separated from one 

 another in detached spots, while, at the same time, impor- 



