176 Coral Reefs and the Glacial Period. [April, 



quote Darwin* — " It is a remarkable fact, strongly insisted 

 on by Hooker in regard to America, and by Alph. de 

 Candolle in regard to Australia, that many more identical 

 or now slightly modified species have migrated from the 

 north to the south than in a reverse direction. We see, 

 however, a few southern forms in the mountains of Borneo 

 and Abyssinia. I suspect that this preponderant migration 

 from the north to the south is due to the greater extent of 

 land in the north, and to the northern forms having existed 

 in their own homes in greater numbers, and having con- 

 sequently been advanced through natural selection and 

 competition to a higher stage of perfection or dominating 

 power than the southern forms." 



The fact is evident, then, that northern forms more encroach 

 upon equatorial and southern regions than southern upon 

 equatorial and northern. Is this due wholly to the greater 

 extent of northern land, as Darwin suggests, or to the south- 

 ward driving force of a cold period having acted more 

 recently than the northward driving force; in other words, 

 does it not point to a glacial period having prevailed in the 

 north more recently than in the south ? 



From this brief consideration of the probable effects of — 

 (1) latest glaciation in northern hemisphere ; (2) latest 

 glaciation in southern hemisphere ; (3) simultaneous glacia- 

 tion in both hemispheres ; what are our results ? Mainly 

 these, I think: — 



1. That an extended ice-sheet in the northern hemisphere 

 would necessarily thrust farther south the equator of heat, 

 and consequently the two isotherms of 6S° within which 

 limit the reef-builders occur. 



2. That this effect would be increased by the southern 

 hemisphere being unduly hot, just in proportion as the 

 northern was unduly cold. 



3. Under these conditions coral reefs would occur in 

 greatest force south of the equator, and, supposing the 

 climate in both hemispheres slowly to approximate to what 

 it is at present, we might now expect to find the oldest — 

 and, therefore, probably the thickest — reefs south of the 

 equator. 



4. Could some sure standard of the growth of coral-reefs 

 be established, we might, by comparing the reefs north and 

 south of the equator, form some idea of the shortest period 

 of time which could have elapsed since an extreme glacial 

 climate prevailed in either the one or the other hemisphere. 



Origin of Species, 5th edition, chap, xi., p. 457. 



