Descent of Man [ 1871]

Chapter XXI - General Summary and Conclusion

  A BRIEF summary will be sufficient to recall to the reader's mind
the more salient points in this work. Many of the views which have
been advanced are highly speculative, and some no doubt will prove
erroneous; but I have in every case given the reasons which have led
me to one view rather than to another. It seemed worth while to try
how far the principle of evolution would throw light on some of the
more complex problems in the natural history of man. False facts are
highly injurious to the progress of science, for they often endure
long; but false views, if supported by some evidence, do little
harm, for every one takes a salutary pleasure in proving their
falseness: and when this is done, one path towards error is closed and
the road to truth is often at the same time opened.
  The main conclusion here arrived at, and now held by many
naturalists who are well competent to form a sound judgment is that
man is descended from some less highly organised form. The grounds
upon which this conclusion rests will never be shaken, for the close
similarity between man and the lower animals in embryonic development,
as well as in innumerable points of structure and constitution, both
of high and of the most trifling importance,- the rudiments which he
retains, and the abnormal reversions to which he is occasionally
liable,- are facts which cannot be disputed. They have long been
known, but until recently they told us nothing with respect to the
origin of man. Now when viewed by the light of our knowledge of the
whole organic world, their meaning is unmistakable. The great
principle of evolution stands up clear and firm, when these groups
or facts are considered in connection with others, such as the
mutual affinities of the members of the same group, their geographical
distribution in past and present times, and their geological
succession. It is incredible that all these facts should speak
falsely. He who is not content to look, like a savage, at the
phenomena of nature as disconnected, cannot any longer believe that
man is the work of a separate act of creation. He will be forced to
admit that the close resemblance of the embryo of man to that, for
instance, of a dog- the construction of his skull, limbs and whole
frame on the same plan with that of other mammals, independently of
the uses to which the parts may be put- the occasional re-appearance
of various structures, for instance of several muscles, which man does
not normally possess, but which are common to the Quadrumana- and a
crowd of analogous facts- all point in the plainest manner to the
conclusion that man is the co-descendant with other mammals of a
common progenitor.
  We have seen that man incessantly presents individual differences in
all parts of his body and in his mental faculties. These differences
or variations seem to be induced by the same general causes, and to
obey the same laws as with the lower animals. In both cases similar
laws of inheritance prevail. Man tends to increase at a greater rate
than his means of subsistence; consequently he is occasionally
subjected to a severe struggle for existence, and natural selection
will have effected whatever lies within its scope. A succession of
strongly-marked variations of a similar nature is by no means
requisite; slight fluctuating differences in the individual suffice
for the work of natural selection; not that we have any reason to
suppose that in the same species, all parts of the organisation tend
to vary to the same degree. We may feel assured that the inherited
effects of the long-continued use or disuse of parts will have done
much in the same direction with natural selection. Modifications
formerly of importance, though no longer of any special use, are
long-inherited. When one part is modified, other parts change
through the principle of correlation, of which we have instances in
many curious cases of correlated monstrosities. Something may be
attributed to the direct and definite action of the surrounding
conditions of life, such as abundant food, heat or moisture; and
lastly, many characters of slight physiological importance, some
indeed of considerable importance, have been gained through sexual
selection.
  No doubt man, as well as every other animal, presents structures,
which seem to our limited knowledge, not to be now of any service to
him, nor to have been so formerly, either for the general conditions
of life, or in the relations of one sex to the other. Such
structures cannot be accounted for by any form of selection, or by the
inherited effects of the use and disuse of parts. We know, however,
that many strange and strongly-marked peculiarities of structure
occasionally appear in our domesticated productions, and if their
unknown causes were to act more uniformly, they would probably
become common to all the individuals of the species. We may hope
hereafter to understand something about the causes of such
occasional modifications, especially through the study of
monstrosities: hence the labours of experimentalists such as those
of M. Camille Dareste, are full of promise for the future. In
general we can only say that the cause of each slight variation and of
each monstrosity lies much more in the constitution of the organism,
than in the nature of the surrounding conditions; though new and
changed conditions certainly play an important part in exciting
organic changes of many kinds.
  Through the means just specified, aided perhaps by others as yet
undiscovered, man has been raised to his present state. But since he
attained to the rank of manhood, he has diverged into distinct
races, or as they may be more fitly called, sub-species. Some of
these, such as the Negro and European, are so distinct that, if
specimens had been brought to a naturalist without any further
information, they would undoubtedly have been considered by him as
good and true species. Nevertheless all the races agree in so many
unimportant details of structure and in so many mental peculiarities
that these can be accounted for only by inheritance from a common
progenitor; and a progenitor thus characterised would probably deserve
to rank as man.