The Economist shows that when a country imports goods the chief means of paying for them is to export other goods.  If this were all, imports and exports would be of equal value:  yet the United Kingdom imports about L400,000,000 annually, and exports about L300,000,000.  Here, then, is a residuary phenomenon of L100,000,000 to be accounted for.  But foreign countries owe us about L50,000,000 for the use of shipping, and L70,000,000 as interest on the capital we have lent them, and L15,000,000 in commissions upon business transacted for them.  These sums added together amount to L135,000,000; and that is L35,000,000 too much.  Thus another residuary phenomenon emerges; for whilst foreigners seem to owe us L435,000,000 they only send us L400,000,000 of imports.  These L35,000,000 are accounted for by the annual investment of our capital abroad, in return for which no immediate payment is due; and, these being omitted, exports and imports balance.  Since this was written the figures of our foreign trade have greatly risen; but the character of the explanation remains the same.

When, in pursuing the method of Variations, the phenomena compared do not always correspond in their fluctuations, the irregular movements of that phenomenon which we regard as the effect may often be explained by treating them as residuary phenomena, and then seeking for exceptional causes, whose temporary interference has obscured the influence of the general cause.  Thus, returning to the diagram of the Price of Tea in Sec. 4, it is clear that generally the price falls as the duty falls; but in Mr. Denyer’s more minutely wrought diagram, from which this is reduced, it may be seen that in 1840 the price of tea rose from 3_s._ 9_d._ to 4_s._ 9_d._ without any increase of duty.  This, however, is readily explained by the Chinese War of that year, which checked the supply.  Again, from 1869 to 1889 the duty was constant, whilst the price of tea fell as much as 8_d._ per lb.; but this residuary phenomenon is explained by the prodigiously increased production of tea during that period in India and Ceylon.

The above examples of the method of Residues are all quantitative; but the method is often employed where exact estimates are unobtainable.  Thus Darwin, having found certain modifications of animals in form, coloration and habits, that were not clearly derivable from their struggle for existence in relation to other species or to external conditions, suggested that they were due to Sexual Selection.

The ‘vestiges’ and ‘survivals’ so common in Biology and Sociology are residuary phenomena.  It is a general inference from the doctrine of Natural Selection that every organ of a plant, animal, or society is in some way useful to it.  There occur, however, organs that have at present no assignable utility, are at least wasteful, and sometimes even injurious.  And the explanation is that formerly they were useful; but that, their uses having lapsed, they are now retained by the force of heredity or tradition.  Either they are not injurious enough to be eliminated by natural selection; or they are correlated with other organs, whose utility outweighs their disutility.