ORNITHOLOGICAL CLUB, 
65 
of immigrants may have been maintained from the earliest times. 
It must be remembered, however, that no precise order of immigra- 
tion can be laid down, even approximately ; for one term in the 
proposition is an absolutely unknown quantity. We know nothing 
of the rate of change that has taken place in any one species. 
Outward circumstances may have acted on one species, so as to 
leave it little changed in a given time, whilst in the same time an- 
other species may have assumed distinctive generic characters. 
Viewing the very peculiar physical characters of these islands when 
contrasted with the neighboring American shores, it would seem 
reasonable that the rate of change demanded of an immigrant 
species would be high ; consequently the origin of the islands need 
not be dated back to a more distant period than seems indicated by 
their volcanic origin.” 
Considered in connection with the subject discussed above, the 
birds of Guadalupe are of extreme interest, since they apparently 
represent a transition stage through which those of the Galapagos 
once undoubtedly passed. Nothing, unfortunately, is known to the 
waiter as to the geological structure of Guadalupe ; the character of 
the modifications presented in its birds, however, point strongly to its 
volcanic origin, and render it extremely probable that the upheaval 
took place at a more recent date than that of the Galapagos. The 
earliest immigrants to this island were probably the ancestors of 
Polyhorus lutosus, which has become completely differentiated in 
plumage but not perceptibly altered in the details of structure,* and 
those of Carpodacus ampins, whose modifications of external struc- 
* The case of this species presents a very curious problem. Its origin from 
P. cheriway, the only species now inhabiting Middle America, and even north- 
ern South America, can scarcely be doubted ; but the modifications which the 
Guadalupe species has undergone tend toward the distinguishing characters of 
the South Ameilcan form (P. tJiarus). The two continental representatives of 
this genus have undoubtedly had a common origin, the differences between them 
coming under the scope of ordinary geographical laws of variation in this 
family, as, at present understood. The differentiation of the Guadalupe form is 
of a most remarkable kind, however, being apparently a partial reversion to the 
features of the Southern form ; but some of the characters which distinguish 
the latter from its Northern analogues are even greatly exaggerated in this North- 
ern insular form ! In this instance, then, the differentiation has been a kind 
of retrocession, with no , change in details of structure, while in all the other 
forms of the island the differentiation has been of the opposite kind, affecting 
the proportions more than the colors. 
