selection, their parts will have become more and more differentiated
and specialised for various functions from the advantage gained by the
division of physiological labour. The same part appears often to have
been modified first for one purpose, and then long afterwards for some
other and quite distinct purpose; and thus all the parts are rendered
more and more complex. But each organism still retains the general type
of structure of the progenitor from which it was aboriginally derived.
In accordance with this view it seems, if we turn to geological
evidence, that organisation on the whole has advanced throughout the
world by slow and interrupted steps. In the great kingdom of the
Vertebrata it has culminated in man. It must not, however, be supposed
that groups of organic beings are always supplanted, and disappear as
soon as they have given birth to other and more perfect groups. The
latter, though victorious over their predecessors, may not have become
better adapted for all places in the economy of nature. Some old forms
appear to have survived from inhabiting protected sites, where they
have not been exposed to very severe competition; and these often aid
us in constructing our genealogies, by giving us a fair idea of former
and lost populations. But we must not fall into the error of looking at
the existing members of any lowly-organised group as perfect
representatives of their ancient predecessors.
The most ancient progenitors in the kingdom of the Vertebrata, at which
we are able to obtain an obscure glance, apparently consisted of a
group of marine animals (32. The inhabitants of the seashore must be
greatly affected by the tides; animals living either about the MEAN
high-water mark, or about the MEAN low-water mark, pass through a
complete cycle of tidal changes in a fortnight. Consequently, their
food supply will undergo marked changes week by week. The vital
functions of such animals, living under these conditions for many
generations, can hardly fail to run their course in regular weekly
periods. Now it is a mysterious fact that in the higher and now
terrestrial Vertebrata, as well as in other classes, many normal and
abnormal processes have one or more whole weeks as their periods; this
would be rendered intelligible if the Vertebrata are descended from an
animal allied to the existing tidal Ascidians. Many instances of such
periodic processes might be given, as the gestation of mammals, the
duration of fevers, etc. The hatching of eggs affords also a good
example, for, according to Mr. Bartlett ( Land and Water, Jan. 7,
1871), the eggs of the pigeon are hatched in two weeks; those of the
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