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14 e NATURAL SELECTION virA Case of Orchid-structure explained by Natural SelectionThere is a Madagascar orchid the Angrsecum sesquipedalewith an immensely long and deep nectary.How did suchan extraordinary organ come to be developed? Mr. Darwin'sexplanation is this. The pollen of this flower can only beremoved by the base of the proboscis of some very largemoths, when trying to get at the nectar at the bottom of thevessel. The moths with the longest probosces would do thismost effectually; they would be rewarded for their longtongues by getting the most nectar whilst on the other;hand,the flowers with the deepest nectaries would be the bestfertilised by the largest moths preferring them. Consequently,the deepest nectaried orchids and the longest tonguedmoths would each confer on the other an advantage in thebattle of life. This would tend to their respective perpetuation,and to the constant lengthening of nectaries and probosces.Now let it be remembered that what we have toaccount for is only the unusual length of this organ. A nectaryis found in many orders of plants and is especiallycommon in the orchids, but in this one case is only it sometimesmore than a foot long.How did this arise ? We beginwith the fact, proved experimentally by Mr. Darwin, thatmoths do visit orchids, do thrust their spiral trunks into thenectaries, and do fertilise them by carrying the polliniaof oneflower to the stigma of another. He has further explainedthe exact mechanism by which this is effected, and the Dukeof Argyll admits the accuracy of his observations. In ourBritish species, such as Orchis pyramidalis, it is not necessarythat there should be any exact adjustment between the lengthof the nectary and that of the proboscis of the insect ;andthus a number of insects of various sizes are found to carryaway the pollinia and aid in the fertilisation. In theAngraecum sesquipedale, however, it is necessary that theproboscis should be forced into a particular part of the flower,and this would only be done by a large moth burying its proboscisto the very base, and straining to drain the nectar fromthe bottom of the long tube, in which it occupies a depth ofone or two inches only. Now let us start from the time whenthe nectary was only half its present length or about six