CHAPTER VI - DIFFICULTIES OF THE THEORY
Summary: the Law of Unity of Type and of the Conditions of Existence
embraced by the Theory of Natural Selection
We have in this chapter discussed some of the difficulties and
objections which may be urged against the theory. Many of them are
serious; but I think that in the discussion light has been thrown on
several facts, which on the belief of independent acts of creation are
utterly obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural
selection is always very slow, and at any one time acts only on a
few forms; and partly because the very process of natural selection
implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a
continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly
graduate away from one part to another. When two varieties are
formed in two districts of a continuous area, an intermediate
variety will often be formed, fitted for an intermediate zone; but
from reasons assigned, the intermediate variety will usually exist
in lesser numbers than the two forms which it connects; consequently
the two latter, during the course of further modification, from
existing in greater numbers, will have a great advantage over the less
numerous intermediate variety, and will thus generally succeed in
supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first only glided through the air.
We have seen that a species under new conditions of life may
change its habits; or it may have diversified habits, with some very
unlike those of its nearest congeners. Hence we can understand,
bearing in mind that each organic being is trying to live wherever
it can live, how it has arisen that there are upland geese with webbed
feet, ground woodpeckers, diving thrushes, and petrels with the habits
of auks.
Although the belief that an organ so perfect as the eye could have
been formed by natural selection, is enough to stagger any one; yet in
the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing
conditions of life, there is no logical impossibility in the
acquirement of any conceivable degree of perfection through natural
selection. In the cases in which we know of no intermediate or
transitional states, we should be extremely cautious in concluding
that none can have existed, for the metamorphoses of many organs
show what wonderful changes in function are at least possible. For
instance, a swimbladder has apparently been converted into an
air-breathing lung. The same organ having performed simultaneously
very different functions, and then having been in part or in whole
specialised for one function; and two distinct organs having performed
at the same time the same function, the one having been perfected
whilst aided by the other, must often have largely facilitated
transitions.
We have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and
independently formed; but when such organs are closely examined,
essential differences in their structure can almost always be
detected; and this naturally follows from the principle of natural
selection. On the other hand, the common rule throughout nature is
infinite diversity of structure for gaining the same end; and this
again naturally follows from the same great principle.
In many cases we are far too ignorant to be enabled to assert that a
part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly
accumulated by means of natural selection. In many other cases,
modifications are probably the direct result of the laws of
variation or of growth, independently of any good having been thus
gained. But even such structures have often, as we may feel assured,
been subsequently taken advantage of, and still further modified,
for the good of species under new conditions of life. We may, also,
believe that a part formerly of high importance has frequently been
retained (as the tail of an aquatic animal by its terrestrial
descendants), though it has become of such small importance that it
could not, in its present state, have been acquired by means of
natural selection.
Natural selection can produce nothing in one species for the
exclusive good or injury of another; though it may well produce parts,
organs, and excretions highly useful or even indispensable, or again
highly injurious to another species, but in all cases at the same time
useful to the possessor. In each well-stocked country natural
selection acts through the competition of the inhabitants, and
consequently leads to success in the battle for life, only in
accordance with the standard of that particular country. Hence the
inhabitants of one country, generally the smaller one, often yield
to the inhabitants of another and generally the larger country. For in
the larger country there will have existed more individuals and more
diversified forms, and the competition will have been severer, and
thus the standard of perfection will have been rendered higher.
Natural selection will not necessarily lead to absolute perfection;
nor, as far as we can judge by our limited faculties, can absolute
perfection be everywhere predicated.
On the theory of natural selection we can clearly understand the
full meaning of that old canon in natural history, "Natura non facit
saltum." This canon, if we look to the present inhabitants alone of
the world, is not strictly correct; but if we include all those of
past times, whether known or unknown, it must on this theory be
strictly true.
It is generally acknowledged that all organic beings have been
formed on two great laws: Unity of Type, and the Conditions of
Existence. By unity of type is meant that fundamental agreement in
structure which we see in organic beings of the same class, and
which is quite independent of their habits of life. On my theory,
unity of type is explained by unity of descent. The expression of
conditions of existence, so often insisted on by the illustrious
Cuvier, is fully embraced by the principle of natural selection. For
natural selection acts by either now adapting the varying parts of
each being to its organic and inorganic conditions of life; or by
having adapted them during past periods of time: the adaptations being
aided in many cases by the increased use or disuse of parts, being
affected by the direct action of the external conditions of life,
and subjected in all cases to the several laws of growth and
variation. Hence, in fact, the law of the Conditions of Existence is
the higher law; as it includes, through the inheritance of former
variations and adaptations, that of Unity of Type.
Previous section | Next section
