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thought—talking rats, rational ants, philosophic dogs, and the like.
But closer analysis shows that these fantasies are impossible. A brain
capable of conceptual thought could not have been developed else-
where than in a human body.
The course followed by evolution appears to have been broadly as
follows. From a generalized early type, various lines radiate out,
exploiting the environment in various ways. Some of these com-
paratively soon reach a limit to their evolution, at least as regards
major alteration. Thereafter they are limited to minor changes such
as the formation of new genera and species. Others, on the other
hand, are so constructed that they can continue their career, generat-
ing new types which are successful in the struggle for existence because
of their greater control over the environment and their greater
independence of it. Such changes are legitimately called "progres-
sive." The new type repeats the process. It radiates out into a
number of lines, each specializing in a particular direction. The
great majority of these come up against dead ends and can advance
no further: specialization is one-sided progress, and after a longer or
shorter time, reaches a biomechanical limit. The horse stock cannot
reduce its digits below one; the elephants are near the limits of size
for terrestrial animals; feathered flight cannot become aerodynamic-
ally more efficient than in existing birds, and so on.
Sometimes all the branches of a given stock have come up against
their limit, and then either have become extinct or have persisted
without major change. This happened, for instance, to the echino-
derms, which with their sea-urchins, starfish, brittle-stars, sea-lilies,
sea-cucumbers, and other types now extinct had pushed the life that
was in them into a series of blind alleys: they have not advanced
for perhaps a hundred million years, nor have they given rise to other
major types.
In other cases, all but one or two of the lines suffer this fate, while
the rest repeat the process. All reptilian lines were blind alleys save
two—one which was transformed into the birds, and another which
became the mammals. Of the bird stock, all lines came to a dead
end; of the mammals, all but one—the one which became man.
Evolution is thus seen as an enormous number of blind alleys, with
a very occasional path of progress. It is like a maze in which almost
all turnings are wrong turnings. The goal of the evolutionary maze,
however, is not a central chamber, but a road which will lead in-
definitely onwards.
If now we look back upon the past history of life, we shall see that
the avenues of progress have been steadily reduced in number, until