E. Ray Lankester has recently
discussed this subject, and he concludes, as far as its extreme complexity
allows him to form a judgment, that longevity is generally related to the
standard of each species in the scale of organisation, as well as to the
amount of expenditure in reproduction and in general activity. And these
conditions have, it is probable, been largely determined through natural
selection.
It has been argued that, as none of the animals and plants of Egypt, of
which we know anything, have changed during the last three or four thousand
years, so probably have none in any part of the world. But, as Mr. G.H.
Lewes has remarked, this line of argument proves too much, for the ancient
domestic races figured on the Egyptian monuments, or embalmed, are closely
similar or even identical with those now living; yet all naturalists admit
that such races have been produced through the modification of their
original types. The many animals which have remained unchanged since the
commencement of the glacial period, would have been an incomparably
stronger case, for these have been exposed to great changes of climate and
have migrated over great distances; whereas, in Egypt, during the last
several thousand years, the conditions of life, as far as we know, have
remained absolutely uniform. The fact of little or no modification having
been effected since the glacial period, would have been of some avail
against those who believe in an innate and necessary law of development,
but is powerless against the doctrine of natural selection or the survival
of the fittest, which implies that when variations or individual
differences of a beneficial nature happen to arise, these will be
preserved; but this will be effected only under certain favourable
circumstances.
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