ALTERNATE GLACIAL PERIODS 421 



given in regard to the plants of that large island. Hence we 

 see that certain plants growing on the more lofty mountains 

 of the tropics in all parts of the world, and on the temperate 

 plains of the north and south, are either the same species or 

 varieties of the same species. It should, however, be ob- 

 served that these plants are not strictly arctic forms ; for, as 

 Mr. H. C. Watson has remarked, "in receding from polar 

 towards equatorial latitudes, the Alpine or mountain floras 

 really become less and less Arctic." Besides these identical 

 and closely allied forms, many species inhabiting the same 

 widely sundered areas, belong to genera not now found in 

 the intermediate tropical lowlands. 



These brief remarks apply to plants alone; but some few 

 analogous facts could be given in regard to terrestrial ani- 

 mals. In marine productions, similar cases likewise occur; 

 as an example, I may quote a statement by the highest 

 authority, Prof. Dana, that "it is certainly a wonderful fact 

 that New Zealand should have a closer resemblance in its 

 Crustacea to Great Britain, its antipode, than to any other 

 part of the world." Sir J. Richardson, also, speaks of the re- 

 appearance on the shores of New Zealand, Tasmania, &c., of 

 northern forms of fish. Dr. Hooker informs me that 

 twenty-five species of Algae are common to New Zealand 

 and to Europe, but have not been found in the intermediate 

 tropical seas. 



From the foregoing facts, namely, the presence of tem- 

 perate forms on the highlands across the whole of equatorial 

 Africa, and along the Peninsula of India, to Ceylon and the 

 Malay Archipelago, and in a less well-marked manner across 

 the wide expanse of tropical South America, it appears 

 almost certain that at some former period, no doubt during 

 the most severe part of a Glacial period, the lowlands of 

 these great continents were everywhere tenanted under the 

 equator by a considerable number of temperate forms. At 

 this period the equatorial climate at the level of the sea was 

 probably about the same with that now experienced at the 

 height of from five to six thousand feet under the same lati- 

 tude, or perhaps even rather cooler. During this, the coldest 

 period, the lowlands under the equator must have been 

 clothed with a mingled tropical and temperate vegetation, 



