54 
Introduction : V ariation. 
be attained wben each S]Decimen is compared with the type — the first-described 
individual — of its species, and a formula representing the difference appended 
to the binomial name. Such a method is however impossible at present for 
practical reasons; there is no means of measuring the differences in question. 
Another reason why individual variation finds so little mention from those who 
have most experience of it is that for a long time a strong effort has been 
made to enforce the recognition of subspecies; the effect of this has been to 
give undue prominence to geographical variation and to divert attention from 
individual and other phases of variation, which form exceedingly difficult factors 
in the study of local races, and are for that very reason disregarded by careless 
or ignorant writers. 
Rancfe of individual variation. — Compared with some other animal forms, 
for instance, certain Beetles, individual variation in birds (other than domesticated 
races) keeps within somewhat narrow limits. The maximum as regards variation 
of plumage is seen in the male Ruff (Machetes pugnax). The extinct Solitaire 
(Pezophaps) of Rodriguez was also extremely variable; its osteological remains 
show that “the variability of colour he (Leguat) had noticed in the females 
— some fair, some brown — was paralleled by the marvellous variability dis- 
played by almost every bone of the skeleton” (Nervton, D. B., 890). Other 
familiar examples of high individual variability among birds are furnished hy 
the Honey Buzzard {Pernis apivorus), the Common Partridge (Perdix cinerea), and 
the Common Crossbill (Loxia curvirostra). The minimum of individual variation 
is perhaps found among certain highly local species of Pigeons, such as those 
of the genera and Osmotreron\ also many Sunbirds, Kingfishers, Parrots, etc., 
seem to be very stable in this respect. The differences separating many species 
of Pigeons are so small that it creates a feeling of surprise to find that large 
series of specimens do not present every sort of intermediate form and other 
modification. Under domestication, on the other hand, the Blue Rock Dove 
has displayed a most remarkable variability. A high degree of individual 
variability is sometimes found among species of birds in which the sexes are 
dissimilar (e. g. Machetes^ Macropygia, Gallns), but the sexes are dissimilar also 
in Ptilopus, Osmotreron, the Sunbirds, and other stable forms, so that this factor 
evidently has no direct infiuence in the matter. 
Some species vary much in the measurements of various parts of the body, 
but little in coloration (e. g. Streptocitta, Amaurornis moluccana); others vary much 
hoth in size of parts and in coloration (e. g. Halcyon chloris, Xanthocmcs favicollis). 
Highly specialized features are generally very variable: for instance, the long 
tails of Streptocitta, Phoenicophaes and Fregata, the long hind claws of Centro- 
coccyx and Hydralector, the long bills of Limosa and Ntmenius, the long tarsus 
of Himantopus; but age also plays an important role in this connection. 
i 
Psychological differences of individuals. — Persons who have closely observed 
or have kept wild species in confinement have often noticed differences of 
