104 THE WOELD OF LIFE 



sion of others. Hence also it has come about that the equa- 

 torial species seem to be better defined — more sharj^ly dis- 

 tin2;nished from each other — than manv of those of the tem- 

 perate and northern zones. They are what Dr. Beccari terms 

 first-grade species, as in the case of Borneo, an island which 

 forms part of what has quite recently been an equatorial con- 

 tinental mass. It is interesting to note that Mr. Th. D. A. 

 Cockerell has arrived at a similar conclusion from his study 

 of the rich fossil flora of Florissant, Colorado, of middle or 

 late Tertiary age, which shows signs of a much milder climate 

 than now prevails there. Many of these plants are of genera 

 now extinct or only found in more southern lands, and this ex- 

 tinction is traceable to the great changes, inorganic and or- 

 ganic, that have since occurred in Xorth America. He says 

 (in a private letter) : 



" There was first the invasion of Old World species via Behring's 

 Straits; then an incursion of S. American forms via Panama; and 

 then the glacial period at the end, crowding and destroying much 

 of the flora and fauna. Since the glacial period in X. America, 

 there has been room for expansion, and hence the very numerous 

 and closely allied species of Aster, Solidago, Senecio, and other 

 jolants, as well as allied species of butterflies of the genera Arg}^nnis, 

 Colias, etc. These are, most of them, not at all on the same footing 

 as the tropical species. ... I think tropical species are better 

 defined than those of the temperate region.'' 



It is a rather curious coincidence that if we take the mean 

 area of the twelve English counties for which I have been 

 able to give the figures, in geographical instead of English 

 miles, the number of square miles will almost exactly equal 

 the average number of their species of flowering plants. Be- 

 low this area, in the mid-temperate zone, the proportion of 

 species decreases, and above it increases, in both slowly at 

 first and with many fluctuations, but afterAvards very rapidly, 

 more especially for the larger areas, so that it requires on a 

 rough average about a two hundred-fold increase of area to 

 double the number of species, and about a thousand-fold to 



