722 Mr. J. Lewis Bonhote : Subspecies [ibis, 
mistaken if they think that the latter alone influence heredity. 
In my book (‘Vigour and Heredity/ p. 173 et seq.) I have 
gone more fully into the causes that may have brought 
about geographical races, but, put briefly, it amounts to 
this—changes of climate and food affect the “ Vigour ” (rate 
of metabolism) of an individual ; externally this shows 
itself primarily in its colour, but also in many other ways. 
The climate acting through the general vigour of the 
individual will affect the nutritive value of the egg (e.g. 
underfeed a breeding bird and see if the chick is not a 
weakling), and thus the individual will tend to produce a 
progeny having a similar vigour to itself and also of a 
similar coloration. 
I am not suggesting, of course, that such a change would 
take place at once, but only in the course of many genera¬ 
tions ; but none the less it does take place, and the conditions 
..brought about by environment are inherited. A further 
proof of! this is that when subspecies which have originated in 
different localities extend their range and meet in a common 
locality (e. g.,' the Meadow-Starlings of North America*), 
they yet retain their subspecific characters. On this line of 
argument it is evident that discontinuous variations can 
never prove true subspecies, since they are not due to environ¬ 
ment. Dimorphic forms may in some cases be due to 
environment ; but these should, 1 think, be given specific 
rank, and, in any case, should not be regarded as subspecies 
and designated by trinomials, since they often occur side by 
side (<?. a., some Skuas, Herons, Fulmar, etc.). My con¬ 
ception of a species as a whole is that there are a number of 
“ factors ” which may or may not have a Mendelian inheri¬ 
tance, but which have a separate and definite inheritance. 
A “unit” species, say the Linnet, will contain a definite 
number of these factors ; another nearly allied “ unit 
species, say the Redpoll, will contain a very large percentage 
of the same factors, but a few different ones, and so on. In 
dimorphic species— e. g., Black-eared and Black-throated 
* See Chapman, Bull. Amer. Mus. N. H. xiii. 1900, p. 318. 
