| Natural selection
acts, as we have seen, exclusively
by the preservation and accumulation of variations, which are
beneficial under the organic and inorganic conditions of life
to which each creature is
at each successive period exposed. The ultimate result will be
that each creature will
tend
to become more and more improved in relation to its
conditions
of life. This improvement will, I think,
inevitably lead
to the gradual advancement of the organisation of the greater number of living beings throughout the world.
But here we enter on a very intricate subject, for naturalists have not defined to each other's
satisfaction what is meant by an advance in organisation.
Amongst the vertebrata the degree of intellect and an approach in structure to man clearly come into play.
It might be thought that the amount of change which the various parts and organs undergo
in their development from the embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva.
Von Baer's
standard seems the most widely applicable and the best, namely, the amount of differentiation of the different
parts (in
the adult state,
as I should be inclined to add)
and
their specialisation for different functions; or, as Milne Edwards would express it, the completeness of the division of physiological labour.
But we shall see how obscure a
subject this
is if we look, for instance, to fish,
amongst which some naturalists rank those as highest which, like the sharks, approach nearest to reptiles;
whilst other naturalists rank the common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ most from the other vertebrate classes.
Still
more plainly we see
the obscurity of the subject by turning to plants, with
which the standard of intellect is of course quite excluded; and here some botanists rank those plants as highest which have every organ, as sepals, petals, stamens, and pistils, fully developed in each flower; whereas other botanists, probably with more truth, look at the plants which have their several organs much modified and somewhat
reduced in number as being of
the highest
rank.
If we look at the
differentiation and specialisation of the several organs of
each being when adult (and this will include the advancement of the brain for intellectual purposes)
as the best standard of highness of organisation,
natural selection clearly leads towards highness;
for all physiologists admit that the specialisation of organs, inasmuch as they perform in this state
their functions better, is an advantage to each being; and hence the accumulation of variations tending towards specialisation is within the scope of natural selection.
On the other hand, we can see, bearing in mind that all organic beings are striving to increase at a high ratio and to seize on every ill-occupied
place in the economy of nature, that it is quite possible for natural selection gradually to fit an organic
being to a situation in which several organs would be superfluous and
useless: in such cases there might
be retrogression in the scale of organisation.
Whether organisation on the whole has actually advanced from the remotest geological periods to the present day will be more conveniently discussed in our chapter on Geological Succession.
But it may be objected that if all organic beings thus tend to rise in the scale, how is it that throughout the world a multitude of the lowest forms still exist; and how is it that in each great class some forms are far more highly developed than others?
Why have not the more highly developed forms everywhere supplanted and exterminated the lower?
Lamarck, who believed in an innate and inevitable tendency towards perfection in all organic beings, seems to have felt this difficulty so strongly, that he was led to suppose that new and simple forms were
continually being produced by spontaneous generation.
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