146 
NATURAL SELECTION 
Vri 
A Case of Orchid-structure explained by Natural Selection 
There is a Madagascar orchid — the Angrsecum sesquipedale 
— with an immensely long and deep nectary. How did such 
an extraordinary organ come to he developed ? Mr. Darwin’s 
explanation is this. The pollen of this flower can only be 
removed by the base of the proboscis of some very large 
moths, when trying to get at the nectar at the bottom of the 
vessel. The moths with the longest probosces would do this 
most effectually ; they would be rewarded for their long 
tongues by getting the most nectar ; whilst on the other hand, 
the flowers with the deepest nectaries would be the best 
fertilised by the largest moths preferring them. Conse- 
quently, the deepest nectaried orchids and the longest tongued 
moths would each confer on the other an advantage in the 
battle of life. This would tend to their respective perpetua- 
tion, and to the constant lengthening of nectaries and pro- 
bosces. Now let it be remembered that what we have to 
account for is only the unusual length of this organ. A nec- 
tary is found in many orders of plants and is especially 
common in the orchids, but in this one case only is it some- 
times more than a foot long. How did this arise 1 We begin 
with the fact, proved experimentally by Mr. Darwin, that 
moths do visit orchids, do thrust their spiral trunks into the 
nectaries, and do fertilise them by carrying the pollinia of one 
flower to the stigma of another. He has further explained 
the exact mechanism by which this is effected, and the Duke 
of Argyll admits the accuracy of his observations. In our 
British species, such as Orchis pyramidalis, it is not necessary 
that there should be any exact adjustment between the length 
of the nectary and that of the proboscis of the insect ; and 
thus a number of insects of various sizes are found to carry 
away the pollinia and aid in the fertilisation. In the 
Angrsecum sesquipedale, however, it is necessary that the 
proboscis should be forced into a particular part of the flower, 
and this would only be done by a large moth burying its pro- 
boscis to the very base, and straining to drain the nectar from 
the bottom of the long tube, in which it occupies a depth of 
one or two inches only. Now let us start from the time when 
the nectary was only half its present length or about six 
