346 ALTERNATE GLACIAL I'EKIODS CirAr. XI. 



arc common to Ncn' Zealand and to Europe, Inti Iiave not beer 

 found in the inlennediate tropical seas. 



From the foregoing^ facts, namely, the presence of temperate 

 forms on the highlands across the whole of equatorial iVfrica, 

 and along the Peninsula of India to Ceylon and the Malay archi- 

 pelago, and in a less ■\vell-raarked mann(T across the wide ex- 

 panse of tropical South America, it appears almost certain that 

 at some former period, no doubt during the most severe part 

 of the Glacial period, the lowlands of these great continents 

 were everyAvhere tenanted under the equator by a considerable 

 number of temperate forms. At tliis ])criod the equatorial cli- 

 mate 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 vmder the same latitudes, 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 luxuri- 

 antly 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, on 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, " Avith forms of the 

 torrid zone harmoniously blended with those of the temperate." 



Now let us see Avhether Mr. Croll's conclusion, that Avhen 

 the northern hemisphere sufTcred fi-om the extreme cold of the 

 great Glacial period, the southern hemisphere Avas actually 

 Avarmer, throAvs any clear light on the present apparently inex- 

 ])licable distribution of A^arious organisms in the temperate 

 jiarts of both hemispheres, and on mountains of the tropics. 

 The Glacial period as measured by ^-ears, 7uust have been very 

 long; and, Avhcn avc remember OA'cr Avhat vast spaces some 

 naturalized plants and animals have spread Avithin a fcAV cen- 

 turies, this period Avill haAC been ample for any amoimt of mi- 

 gration. As the cold became more and more intense, Ave knoAV 

 that Arctic forms iuA'aded the temperate regions ; and, from the 

 facts just given, there can hardly be a doubt that some of the 

 more A'igorous, dominant, and widest-spreading temperate forms 

 actually then iuA'aded the equatorial loAvlands. The inhabitants 

 of these lowlands Avould at the same time migrate to the tropi- 

 Ciil and sub-tropical regions of the south, for the southern hemi- 



