240 
MR. J. H. GURNEY ON THE TENDENCY IN BIRDS 
T. 
ON THE TENDENCY 
IN BIRDS TO RESEMBLE OTHER SPECIES. 
By ,T. H. Gurney, F.L.S., Y.-P. 
Read 29 th September, 1890. # 
I believe I am not the only observer who has long thought that 
among Birds, both British and foreign, there is a tendency in 
species to vary now and again in colour of plumage, so as to 
resemble more or less other allied species, without this occasional 
“ sport ” in any way invalidating their distinctiveness as separate 
forms worthy of binomial nomenclature. 
Thoughts such as these must have passed through the mind of 
almost every collecting naturalist, and puzzled him in some branch 
of zoology, if not in ornithology especially. It remains to he 
seen whether the theory has anything in it which will bear the 
test of examination, as, if it can he proved, it is of immense 
importance to ornithologists. 
It is a tendency distinct from climatal variation, hut which has 
nevertheless, if it can be proved, probably been answerable for the 
creation of some of the many subspecies given to the world 
of late years by writers. That such a tendency does exist, seems 
extremely probable, though at present our eyes are too veiled to 
understand it; and that it is something wholly different from the 
absence of colouring matter which makes albinism, or the excess 
of pigmentation which makes melanism and erethryism, is clear. 
The existence of wild hybrids greatly complicates this question of 
resemblance by variation in Birds, for it is almost impossible to 
prove that the twenty variations to be enumerated (and a few 
others of nearly the same kind recorded in Suchetet’s ‘Oiseaux 
Hybrides ’) were not due to crossbreeding. There can be no doubt 
whatever that crossbreeding goes on in a state of nature where 
species overlap or find a scarcity of hens of their own kind, and 
also occasionally where this is not the case. Illustrative of the 
