721 
1921 .] and their part in Evolution . 
subspecies, and were designated in our “ system ” by a tri¬ 
nomial. In my own opinion the binomial species name is the 
only one that should appear as a heading in books and papers, 
and the subspecies should be given as a sub-heading— this, 
by the way, as my present object is not to deal with trivial 
details, but to get at what subspecies are and the part they 
play in evolution. 
It is generally agreed, though the rule is frequently 
honoured in the breach, that trinomials should only be given 
to geographic forms—that is to say, to variations due solely 
to the influences of the climatic conditions of their habitat, 
and it is reasonably stretched to include island varieties; but 
it must not be forgotten that in an island race, “ isolation 5 
forms an additional factor which has influenced the separa¬ 
tion ; and in a closely-lying group of islands where each 
island has a distinct form though the climate is similar, 
“ isolation ” has probably been the chief factor in separating 
the race *. I will now consider Messrs. Lowe & Praed’s 
letter (‘ Ibis,’ p. 344), in which at the beginning they strike 
the right note by asking “ of what scientific value are these 
variations ? ” I will not deal at length with the rest of this 
interesting letter, but merely note two points—they imply 
that subspecies are of two kinds : (1) due to “discontinuous ” 
variation, (2) due to environment. I had always understood 
that a true subspecies was always supposed to be restricted 
to the latter cause, and certainly think it should be so. 
Secondly, they imply that variations (subspecies) caused by 
environment cannot be inherited ; and hence it would follow 
that a true subspecies could have had no influence on 
evolution, and could never develop into a new species. 
Unfortunately, neither of these authors can have had any 
practical experience of breeding birds or animals, or they 
would realize how very small variations can be intensified 
and fixed from a purely selective, as apart from any creative, 
agency. They are quite right in carefully distinguishing 
between continuous and discontinuous variations, but they are 
* The same conditions occur not unfrequently on land where races are 
separated by tracts of country unsuitable to them. 
