144 
DARWINISM 
CHAP. 
would he ample beneficial variations; if a darker colour or 
a longer beak or wing were required, there are always a con¬ 
siderable number of individuals darker and lighter in colour 
than the average, with longer or with shorter beaks and wings, 
and thus the beneficial variation must always be present. And 
so with every other part, organ, function, or habit; because, as 
variation, so far as we know, is and always must be in the two 
directions of excess and defect in relation to the mean amount, 
whichever kind of variation is wanted is always present in some 
degree, and thus the difficulty as to “ beneficial ” variations 
occurring, as if they were a special and rare class, falls to the 
ground. No doubt some organs may vary in three or perhaps 
more directions, as in the length, breadth, thickness, or curva¬ 
ture of the bill. But these may be taken as separate varia¬ 
tions, each of which again occurs as “ more ” or “ less and thus 
the “ right ” or “ beneficial ” or “ useful ” variation must always 
be present so long as any variation at all occurs ; and it has not 
yet been proved that in any large or dominant species, or in 
any part, organ, or faculty of such species, there is no variation. 
And even were such a case found it would prove nothing, so 
long as in numerous other species variation was shown to exist; 
because we know that great numbers of species and groups 
throughout all geological time have died out, leaving no 
descendants ; and the obvious and sufficient explanation of this 
fact is, that they did not vary enough at the time when varia¬ 
tion was required to bring them into harmony with changed 
conditions. The objection as to the “right” or “beneficial” 
variation occurring when required, seems therefore to have no 
weight in view of the actual facts of variation. 
Isolation to prevent Intercrossing. 
Most writers on the subject consider the isolation of a 
portion of a species a very important factor in the formation 
of new species, while others maintain it to be absolutely 
essential. This latter view has arisen from an exaggerated 
opinion as to the power of intercrossing to keep down any 
variety or incipient species, and merge it in the parent stock. 
But it is evident that this can only occur with varieties which 
are not useful, or which, if useful, occur in very small 
numbers; and from this kind of variations it is clear that 
