12-L S0MMERS — FLORA OF NOVA SCOTIA AND COLORADO. 



opposite on either side, constitutes a very important factor when we 

 wish to form an estimate of the relative physical peculiarities or 

 conditions of such places. 



Notwithstanding what has just been stated, and the proof which 

 the synopsis gives of a more copious Flora to Colorado than our 

 own, a flora too which numbers southern species exotic to us, it yet 

 includes nearly one-third of our indigenous species, also many 

 genera which have closely allied species. 



Taking the position that every boreal species in southern situa- 

 tions must have passed thence, as every southern species found north 

 must have migrated there, we may justly claim these boreal species 

 in the Coloradian flora as our own, and endeavour to account for 

 their emigration to that locality as well as for their maintenance or 

 continuance outside of their proper zone. 



To this end I will ask you to follow me for a short space into 

 the region of theory, since the elucidation of these points in our 

 discussion can be arrived at in no other way. 



Taking first the question of continuance we find the species 

 referred to confined more or less to the elevated parts of the region 

 which they inhabit, these regions are from the peculiar physical 

 conditions before mentioned exposed to greater vicissitudes of climate 

 than that of our own, the climate there being excessive, having great 

 extremes. Our extremes being less gives us a more moderate con- 

 dition of climate, the probabilities are however, that between the 

 extremes of the excessive climate of the elevated regions of Colorado 

 we have a mean or middle condition, corresponding more or less 

 to our own, and therefore offering favourable conditions for the 

 growth and continuance of such boreal species as have there esta- 

 blished themselves. 



In addition to this we have the inherent property existing in 

 many plants of living under conditions which, though apparently 

 nnfavourable, are yet not too far removed from those of their original 

 surroundings. 



The problem of plant dispersion while highly interesting, is yet 

 surrounded by difficulties of such a nature as would at the first glance 

 seem to render its elucidation impossible. Nevertheless, many of 



