18 
POPULAR SCIENCE REVIEW. 
on these differences that natural selection acts. Hence it 
happens that the transformation of one kind into another is a 
very slow and gradual process, because it has to be accom- 
plished by a series of very small steps. A long step cannot be 
taken unless it is more to the advantage of the individual than 
a short step in the same direction, because it is certain that 
many more individuals will be born in any given generation 
with the small than with the large variation ; and, unless the 
large one has some direct advantage over the small, the mere 
superiority of numbers will give the victory to the latter. Let 
us illustrate this by an example. Suppose a flower, such as 
the Angrcecum Sesqui'pedale of Madagascar, with a very deep 
nectary, and a supply of nectar at the bottom of it. This 
can only be reached by a moth with a very long proboscis. 
Suppose also that this nectary has, from any cause, a continual 
tendency to lengthen in successive generations. It is evident 
that moths that happen to be born with probosces longer than 
the average will have an advantage over those that are born 
with them shorter. They will have at least, other things 
being equal, one more flower to feed on, and so have a better 
chance for life. Natural selection will therefore operate to 
produce a Kind of moths with long probosces. But it will not 
give any preference to a proboscis longer than is required for 
that special purpose. A proboscis which has an inch to spare 
would not be a bit more useful than one which could just 
drain the nectar and no more. And while many moths would 
be born with the slight additional length necessary for this, 
few or none would be born with tie proboscis an inch longer. 
Such moths would be monstrosities, and monstrosities are always 
rare. And there would be no cause at all tending to perpetuate 
such a monstrosity and to counteract the universal tendency in 
all such cases to return, if unchecked, to the normal type — a 
tendency which is, in point of fact, simply another expression 
of the perpetual effort, which all life manifests, to bring itself 
into absolute harmony with all around it. The music of the 
spheres will not tolerate a discord ; if a half-note too high 
or too low can be caught occasionally by the listening ear, it is 
soon swept out and lost in the full strong current of advancing 
sound. The' office of natural selection is to maintain this con- 
cord, and it does it by favouring those slight variations which, 
by bringing their possessor more into harmony with the world 
around, give him an instant advantage over his fellows. It 
does not favour any larger variations ; it has no forecasting 
eye to the possibilities of any future advantage to be derived 
from them. 
Now let us apply this principle once more to the case of 
man, and in so doing let us pass from an external and super- 
