5^ Introduction: Variation. 



in a certain direction (as a complete ceasing from all variation, even under 

 unaltered conditions, is inconceivable in the course of the propagation of organic 

 forms), or they may be caused by local influences. For some cases the former 

 assumption appears unavoidable; for other cases there is satisfactory evidence 

 of the effect of local influences, though the exact nature of these latter is almost 

 always uncertain; as a rule, probably, both causes operate together, but it very 

 rarely happens that an opinion either way is permissible at present. In illu- 

 stration of independent development in the same direction it may be mentioned 

 that the genus Lurkulus has produced two species with very similar red crowns 

 in Ceylon and in the Southern Philippines; and a corresponding distribution of 

 markings is seen in tHe plumage in the two forms of Celebes [L. stigmatus) and 

 Sooloo {L. houapartei), which are not closely related to one another (see p. 1(J3 

 et seq.). Local influences are sufficiently indicated when a number of species 

 are found to vary in the same place in a corresponding manner; for instance, 

 two phases of modification have been detected by Mr. Allen among North 

 American birds and are recapitulated by Professor Newton (Diet. B. 1S96, 

 p. 1005) as follows: there is "a general increase of intensity toward the south 

 and development of dark markings at the expense of the light intervening 

 spaces, so that of brightly-coloured species southern individuals are the most 

 brightly coloured, and some tints, which to the northward cannot be called 

 brilliant, become vivid in a lower latitude. In respect of longitude Variation 

 occurs with like regularity, the differences appearing to hold a direct relationshiji 

 to the humidity of the climate. Thus on the dry plains of the middle and 

 western parts of the continent birds have a pallid complexion, while on the 

 Pacific slope they resume nearly the tints of the eastern form, though further 

 to the northward, in the rainy belt that extends along the coast of British 

 Columbia, they acquire a depth of colour far in excess of that which they 

 display on tlie Atlantic border."') In such cases the direct influence of climate 

 upon the colours appears to be proved. 



The following instances of correlated geographical variation are noticeable 

 among the birds of the Celebesian Province. 



Increase of size in the Sangi and Talaut Islands. — The local species or races 

 of Sangi and Talaut having their nearest affinities with species on the mainland 

 of Celebes are: 



') This quotation, which we have reprinted, is only from a resume of Mr. Allen's original memoir: 

 "On the Mammals and Winter Birds of East Florida" (Bull. Mus. Comp. Zool. Cambridge, 1870—1, 11, pp. 161 

 — 450, plates IV — VIII, which should be consulted here, chiefly pp. 2:i9 sq., where he says: "Causes of 

 Climatic Va ration. — ... The southward increase in depth of color and in iridescence in birds specifically 

 identical coincides also with the general increase in brilliancy of color in birds, taken as a whole, in the lower 

 latitudes (as well as in insects and animals generally), the maximum being reached in the tropics. — The 

 longitudinal Variation, or the westward increase in color, seems to be also coincident with the increased humidity 

 to the westward, tlie darker representatives of any species occurring where the annual rainfall is greatest and 

 the palest where it is least . . ." 



