H$ Alternate Glacial Periods Chap. xh. 



JO 



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 animals. 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 reappearance on the shores of New Zealand, Tasmania, &c, 

 of northern forms of fish. Dr. Hooker informs me that twenty- 

 five species of Algse 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 temperate 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 latitude, 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, like that described by Hooker as 

 growing luxuriantly at the height of from four to five thousand feet 

 on the lower slopes of the Himalaya, but with perhaps a still 

 greater preponderance of temperate forms. So again in the moun- 

 tainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann 

 found temperate European forms beginning to appear at the height 

 of about five thousand feet. On the mountains of Panama, at the 

 height of only two thousand feet, Dr. Seemann found the vegetation 

 like that of Mexico, " w T ith forms of the torrid zone harmoniously 

 blended with those of the temperate." 



Now let us see whether Mr. Croll's conclusion that when the 

 northern hemisphere suffered from the extreme cold of the great 

 Glacial period, the southern hemisphere was actually warmer, throws 



