president’s address. 
119 
however wide the extreme forms may be apart, they are connected 
together by a series of intermediate forms, or to use the American 
technical term, they intergrade. The other point is the extreme 
narrowness of the line which frequently divides one species from 
another where the species are isolated on islands, but where the 
difference, however small, must bo regarded as specific, because, 
however narrow the line, it is a hard and fast one, or again, to 
use the American technical term, they do not intergrade. 
I need not occupy your time in giving details of the many species 
which illustrate the first point. You will find some of the most 
interesting explained in my recent book on the Birds of the Japanese 
Empire. It may however bo worth while to dwell at more length 
on somo of the most interesting examples of the second point 
which have recently come under my notice. 
If I wero askod to enumerate tho five most important factors in 
the origin of species, I should say that the first was the tendency 
of species to vary ; the second, the hereditary character of the 
variations; the third, the preservation of each hereditary variation 
by preventing the species possessing it from mixing with those that 
do not possess it, or to express it in a single word — Isolation ; 
tho fourth, Isolation ; and the fifth, Isolation. 
I am .at present engaged upon a work on tho Birds of Polynesia, 
than which there is no part of the world more adapted to the study 
of the effects of isolation. Polynesia consists of a dozen or more 
Archipelagoes, most of them of volcanic origin, but some composed 
partly of older rocks which may have been above the level of the 
sea for ages. It is not known that there is any evidence of the 
remains of a larger continent. There are no mammals on any of 
the Polynesian Islands, except Eats, Cats, and Pigs, which have 
been recently introduced by man. It is true that there are also 
no mammals in New Zealand ; but in that country the remains 
of large birds which have only recently become extinct, seem 
to suggest a former much greater extent of land where such 
species could have lived, and have become differentiated into 
several species. 
There are no archaic birds to be found in Polynesia, or perhaps 
