1004 VARIATION 



perceptible to the carefully-trained eye.^ It is a trite remark that in a 

 flock of sheep the ordinary man sees nothing to distinguish one animal 

 from another, while the shepherd knows each unfailingly, and those 

 who look after birds kept in captivity are soon able to do the same 

 in regard to their charges, though both shepherd and bird-keeper 

 Avould often find it impossible to point out wherein the difference 

 lay. Yet because the difference cannot be expressed in words, its 

 existence is not to be denied, and indeed for all practical purposes 

 we may assume its existence except in rare cases. Thus, believing 

 Variability to be general, the question naturally arises as to its 

 limits, if it has any. Some there are who would boldly declare it 

 to be in one sense boundless, and others would define its limits as 

 geographical. Much is to be said for this last point of view, which 

 was that taken bv some of the earliest investi orators of Variation ; but 

 then it must be admitted that those who adopt it have a very summary 

 way of treating the subject, though it is eminently practical and 

 perhaps at present indispensable. 



When a definite structure or coloration of any form is observed 

 to be associated with a definite area, and to cease on that area being 

 overpassed, the systematist will generally say at once that we have 

 two distinct species of the form ; but, on the other hand, he is often 

 puzzled how to regard a form that ranges over an area, mostly 

 large, at one end of which it exhibits certain well-marked characters 

 which gradually vanish as the centre is approached, and are as 

 gradually replaced by different but equally well-marked characters, 

 imtil at the other end it generally, though not always, assumes a 

 wholly distinct appearance, the intervening space being thus occupied 

 by individuals more or less intermediate between the extremities of 

 the series. The reflective natiu-alist will perceive the probability of 

 both these categories being reducible to the same principle — only 

 in the latter case the Variation is continuous and in the former 

 discontinuous ; but it has taken ornithologists a long while to 

 recognize this probability. 



Conditions such as these are furnished in cases far too numerous 

 to name,^ and their proper recognition and full appreciation (if that 



specifically characteristic, so that it was utterly impossible to declare the limits 

 of individual modification, while the Variability did not wholly depend on age 

 or sex {Phil. Trans. 1869, pp. 330, 331). Subsequent examination of a still 

 larger series of specimens confirmed the former statement {pj}. cit. vol. 168, p. 

 451). Yet all this amount of Variation was exhibited by the individuals of a 

 single species, confined to a small island, and apparently all living at about the 

 same period and under the same conditions. 



^ The more minutely a specimen is described the less chance there is ot 

 another specimen being found to agree with it. Hence the value of a proper 

 diagnosis, in the old sense of the word, compared with a description — a fact 

 which some modern ornithologists are apt to overlook. 



" They are mostly found among the Oscincs, but possibly because that group 



