PROF. T. H. HUXLEY ON THE GENTIANS. 123 
localities. I do not think it probable that the process of modifi- 
cation and the materials it works upon would be so similar in 
widely different localities as to give rise to the close similarities 
which lead us to group individuals in the same species; but the 
poly geny of genera, and still more of larger groups, appears to 
me to be highly probable *. 
We are very much in the habit of tacitly assuming that because 
certain plants and certain animals exist only under certain 
climatal conditions, there is something in what we vaguely call 
the “ constitution " of the plants or animals which binds them to 
these conditions, and renders it impossible for them to live 
elsewhere. I wish we could get rid of this word “ constitution ;"' 
for I take it to be one of the many verbal anodynes by which the 
discomfort of ignorance is dulled. If it means any thing definite, 
it merely signifies that there is some morphological or physiolo- 
gical impediment to the existence of the plant or animal, outside 
the defined conditions; and our business is to find out what that 
impediment is. When I was at Arolla I was extremely asto- 
nished by the distribution of two very common species of Epilo- 
bium (E, spicatum and E. Fleischeri), which were flowering at the 
same time. There was any quantity of the latter among the 
boulders of the bed of the stream at the bottom of the valley; 
but nowhere else. And there was any quantity of E. spicatum 
growing on the lateral walls of the valley, from 30 or 40 feet 
above its bed upwards, but nowhere else. I used to amuse 
myself by looking for trespassers of either species in the pro- 
vince of the other; but I never could find any. Everybody 
knows that the seeds of the Epilobia are abundant and provided 
with special aids to distribution by the winds; and there cer- 
tainly is no failure of breezes in the valley. Every year, there- 
fore, millions of seeds of each species must be scattered over the 
territory of the other. Why does one thrive, and the other die? 
To say this depends on the “ constitution” of the two spe- 
cies, is simply to wrap up the fact in another form of words. 
And I can conceive of no investigation more likely to lead to 
results of far-reaching importance, than that which should get to 
the bottom of this, or any analogous case. l 
Finally, I may put another question. Is anybody in a posi- 
* I find weighty remarks on this topic in Engler's “ Versuch einer Entwicke- 
lungsgeschichte der Pflanzenwelt," 2 Theil, p. 318. 
