116 PROF, T. H. HUXLEY ON THE GENTIANS. 
some are climbers, are problems hardly to be solved by the 
consideration of the causes which have brought about the mo- 
dification of the corolla. But I have little doubt that, with 
larger knowledge, analogous causes will be found to be operative 
in all these cases. One of the great lessons which Darwin has | 
taught us is faith in the doctrine of sufficient causes; and conse- 
quently hesitation in assuming that any structure, however slight 
or unimportant in appearance, is devoid of significance in relation 
to either present or past conditions of existence, the chiefest of 
these being the struggle for existence with competitors, while 
climate and station probably occupy a very secondary place. 
Even in respect of geographical distribution—upon which 
climate and station are usually assumed to exercise so great an 
influence,—facts which have come under my notice in studying the 
Gentians have led me to be a little sceptical as to the extent of 
that influence. 
At Arolla I never met with a specimen of Gentiana acaulis 
anywhere except in the region between the pine-woods and the 
snow-line. Yet this same species grows so freely in some parts 
of Southern England, as to be used for the borders of beds in a 
kitchen-garden*. The genus Erythrea, which is notorious for 
the slightness of the differences between its “species,” is of 
world-wide distribution. It occurs all over Europe, in the Sin- 
aitic Desert, in Egypt, in Hindostan, in the hottest parts of Aus- 
tralia, and in the moist temperate climate of New Zealand. 
Mr. Gunn, in à note appended to specimens of Gentiana mon- 
tana in the Kew Herbarium, says that this species occurs every- 
where, from the shore to the summit of the mountains. Gentiana 
campestris is said by Hooker and Arnott to be “abundant in 
Scotland, especially near the sea." It was no less abundant at 
Arolla from 6400 feet to the snow-line. 
In studying, with some care, the geographical distribution of 
various large and widespread groups of closely allied animals, 
such as the Canide, the Astacomorpha, and freshwater Fishes, I 
have been much impressed by the necessity of a most minute 
study of their morphology as a preliminary to any attempt to 
deal with the facts of distribution. I think there is no greater 
mistake than to suppose that distribution, or indeed any other 
large biological question, can be studied to good purpose by those 
* Lady Elizabeth Oust informed me that this was the case at Cobham Park 
in her childhood. 
