422 G. R. Wieland — Polar Climate in Time. 



trated. According to C. Hart Merriam* nine species of plants 

 brought back from Lady Franklin Bay by Lieut. Greely also 

 occur on the bleak and storm beaten summit of San Francisco 

 Mountain in Arizona, hundreds of miles south of any other 

 known station. f These plants grow at an altitude of 3,500 

 meters and their seeds were either carried southward by birds, 

 or they are a far southern remnant of a once wider habitat 

 during glacial times, which is more probable. Beneath tlie 

 summit where these isolated species grow are found on a small 

 scale successive realms of animals and plants recapitulating in 

 a way the great life belts that can be distinguished in passing 

 from the mountain base to the Arctic Ocean. JSTow it is very 

 clear that in the event of secular or perchance local decrease 

 in temperature the Arctic forms at the top of San Francisco 

 Mountain would immediately step into the places left vacant 

 down the slopes by killing or weakening frost. Conversely, 

 if melior conditions were to begin from any cause to rule at the 

 summit, owing to increased seasonal heat or dryness, overlapping 

 especially the reproductive season, or for other causes, this far 

 southern skirmish line of the advancing northern flora would 

 doubtless be beaten back or destroyed. Every agriculturist 

 knows this -principle and avails himself of it. Experience has 

 everywhere taught that for any given locality northern varie- 

 ties of plants (and animals) are the hardier, and that southern 

 stocks are weaker and even impossible of successful introduction . 

 Hence for any given isotherm as viewed at the present time the 

 maximum of easy conditions of growth always lies somevjJiere 

 to the south. And it appears that among other effects, where 

 forms of life do succeed in holding out against more and more 

 stringent conditions there is especially a resulting increase of 

 fertility, while on the other hand, where such forms after being 

 inured to rigors are transplanted, or naturally make their way 

 into far more favorable conditions, there results a more robust 

 growth and a decrease in fertility. This would be one 

 prime reason why the southward stress due to secular diminution 

 in heat would be the more readily taken advantage of. 



Evidently then from the record of the past, the vast majority 

 of southern types are known to have made their w^ay from the 

 north, and at the same time the present organic facies and 

 everyday knowledge shows that life utterly fails to make its 

 way northward again and that southward stress is ever present. 



* Biological survey of the San Francisco Mountains, U. S. Dept. of Agri- 

 culture. North American Faunae. Bulletin No. 3, Washington, 1890. 



f List of plants growing both on the shores of Lady Franklin Bay, and at 

 the summit of San Francisco Mountain, Arizona : 



Androsace septentrionalis ; Arenm^ia verna ; Cerastium alpinum ; Cystop- 

 teris fragilis ; Saxifraga caespitosa ; Saxifraga nivolis ; Oxyna digyna ; 

 Trisetum subspicatum. 



