14 e NATURAL SELECTION vir 



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 be 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 ? 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 

 Angraecum 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 



