rotundifolia, Ranunculus sylvaticus, Galium cruciatum,
and others. It is less marked in the case of Thymus serpyllum and
Geranium sylvaticum; while in Rosa alpina and Erigeron alpinus no
difference is observable.[2]

In the following cases a difference of intensity of colour is,
according to Kerner ("Pflanzenleben," 11. 504), especially
noticeable:-- _Agrostemma githago, Campanula

[1] _Proc. Royal Dublin Society_, 1893.

[2] G. Bonnier, quoted by De Varigny, _Experimental Evolution_,
p. 55.

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pusilla, Dianthus inodorus (silvestris), Gypsophila repens, Lotus
corniculatus, Saponaria ocymoides, Satureja hortensis, Taraxacumm
officinale, Vicia cracca, and Vicia sepium._

To my own observation this beautiful phenomenon has always
appeared most obvious and impressive. It appears to have struck
many unprofessional observers. Helmholtz offers the explanation
that the vivid colours are the result of the brighter sunlight of
the heights. It has been said, too, that they are the direct
chemical effects of a more highly ozonized atmosphere. The latter
explanation I am unable to refer to its author. The following
pages contain a suggestion on the matter, which occurred to me
while touring, along with Henry H. Dixon, in the Linthal district
of Switzerland last summer.[1]

If the bloom of these higher alpine flowers is especially
pleasing to our own aesthetic instincts, and markedly conspicuous
to us as observers, why not also especially attractive and
conspicuous to the insect whose mission it is to wander from
flower to flower over the pastures? The answer to this question
involves the hypothesis I would advance as accounting for the
bright colours of high-growing individuals. In short, I believe a
satisfactory explanation is to be found in the conditions of
insect life in the higher alps.

In the higher pastures the summer begins late and

[1] The summer of 1892.

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closes early, and even in the middle of summer the day closes in
with extreme cold, and the cold of night is only dispelled when
the sun is well up. Again, clouds cover the heights when all is
clear below, and cold winds sweep over them when there is warmth
and shelter in the valleys. With these rigorous conditions the
pollinating insects have to contend in their search for food, and
that when the rival attractions of the valleys below are so many.
I believe it is these rigorous conditions which are indirectly
responsible for the bright colours