ON THE PHEASANT OF ST. HELENA. 227 
and doubtless having been fed for that length of time on some- 
what different food, one might expect to find that they had become 
to some extent differentiated from the original form. 
By the kindness of Col. Edmund Palmer I have had an 
opportunity of comparing the series of St. Helena Pheasants 
with a series of the same species from Siberia, China, the Corea, 
and Formosa. The Pheasant inhabiting the last-named island 
is so much paler in the ground colour of the upper back and 
flanks that Mr. Elliot described it as a distinct species under 
the name of Phasianus formosanus (Proc. Zool. Soc. 1870, 
p- 406). There is, however, no doubt that it can only claim 
to be subspecifically distinct, as examples from the Corea are 
intermediate in colour. The St. Helena birds, though they 
have also enjoyed the advantages, or disadvantages, of island 
life for so many centuries, have retained the typical colour, 
and, after a careful examination, the only difference that I have 
been able to discover between them and Siberian examples is 
so slight that it might almost escape detection. In the Siberian 
Pheasant the feathers which form the white ring round the neck 
haye narrow black margins on the nape. In the St. Helena 
Pheasant these black margins are not quite so narrow, and are 
also more or less distinctly traceable on the white feathers of the 
sides and front of the neck, as well as on the nape. 
This interesting fact must not be regarded as tending in any 
way to lessen our estimate of the importance of isolation in 
producing differentiation. Strictly speaking isolation does not 
directly produce differentiation. Logically stated, the facts are 
these: individual variation is produced by a variety of causes, 
change of climate, change of food, change of habits, the action 
of the two former being probably direct, and principally affecting 
colour, that of the latter being indirect and affecting the 
structure of the various organs, whilst they are in a more or 
less plastic condition between the birth and arrival at maturity 
of the animal, the organs being developed by use or degraded 
by disuse. These variations are hereditary, and the part that 
isolation plays is that of preventing interbreeding from ob- 
structing the action of Natural Selection in causing the rapid 
accumulation or continuation of the variations which are 
advantageous to the species. The inference to be drawn is that 
in the life of even so variable a bird as a Pheasant, and even 
