IV INTRODUCTION.
This assumption, if persistently followed out, would of course account for the greatest
diversities t h a t exist in Nature, if sufficient time were allowed; but i t is obvious that, however
far it may account for multiplication and modification of species, it cannot account for ultimate
origin; for it presupposes life: this puts one limit on its action, and the discoveries in
geology, though the most diverse opinions have been held respecting their real force, seem
t o require a still further limitation. The laws of generation apply equally to the vegetable
kingdom; and here too we find the universal individual differences; also the close approximation
of forms, and apparent gradation from group to group, which characterizes the internal
arrangement of each kingdom, is equally perceptible on the line of j u n c t i o n ; and the progression
of form from vegetable to animal is as complete as from fish to reptile, or from bird
to mammal. If these gradations are held to be good evidence of a common origin in the
one case, they must be equally so in the other; and the animal world, with the vegetable,
must be referred to a single stock; and on t h e other hand, if they can be shown to be compatible
with independent origin in t h e one case, we may assume that they are so also in the
other. From geology, we learn that both animal and vegetable life appeared in the oldest
fossilifcrous stratum, and that, though the animal forms came later and in less variety than
t h e vegetable, the intermediate stages have neither been discovered, nor has there been any
break in the continuity of the stratum in which modification may be assumed to have taken
place unrecorded; and further, as the conditions were evidently favourable for preserving
their remains had they existed, t h e inference may fairly be drawn that they did not or, in
other words, t h a t the origins of t h e two kingdoms were independent. If this be t h e case,
t h e minute gradations of form still existing between animals and vegetables is shown to be
no proof of common origin; and we may go one step further, and assume that it may also be
no proof in the case of the various orders, families, and genera of the animal kingdom.
I f we follow out this idea through the succeeding fossil-bearing strata, we find, speaking
in general terms, t h a t the great divisions of the animal kingdom appear on the stage in succession,
and that each succeeding era is more or less distinctly characterized by the advent of
a new and higher order coexisting with the preceding forms, but with no evidence of minute
gradations of forms connecting them. In most instances these new eras are preceded by
blank intervals of unknown duration; and in them the intermediate forms might have been
assumed to have existed and been destroyed, had we not the evidence of the period of the
first introduction of animal life to guide us in assuming the contrary.
Again, we have t h e fact that throughout the whole period of deposition of fossils up to the
present day the animal and vegetable kingdoms have existed side by side, and the gradations
of form between them, though fine, are imperfect, and were no finer or more gradual in the
Upper Silurian period than they a r e at the present day. These various results would seem
to indicate that the law of generation does not entirely fulfil the conditions of the problem,
and that there are other forces controlling and counteracting this multiplication of individual
differences, and that the law, instead of being a single force, is rather the resultant of a
number of forces which, compensating each other, produce the harmonious whole that we
see around us. The minute gradations of species points to certain amount of affinity; the
INTRODUCTION. v
evidence of geology tends to show that this affinity is not universal; the question is, what is
t h e limit %
The only way open to us for investigating the nature of these forces is through analysis
and comparison of the effects produced by them. And what are these effects? A vast
variety of forms of organisms of every shade and degree, and this variety of form not only
before our eyes at the present day, b u t reaching backward into boundless time as far as the
earth itself can carry t h e record—still an almost endless variety—showing, beyond all question,
that, as far back at least as geology can reach, whatever be t h e duration of that period, the
organic portion of the surface of the globe, throughout its stages of progression, has presented
a countless variety of forms. Further, as an effect, we have the fact of universal
individual differences—that is, t h a t no two individuals of any species can be held to be absolutely
identical (we may assume t h i s ; for it is a natural deduction from what we see to be
the case among the higher forms, and, as far as we have any means of testing, it has proved
to be so with the lower); and these differences vary in degree and in kind. Lastly, notwithstanding
this vast amount of variety, individuals of the same stock retain their identity to
the furthest limit to which we are able to trace them, and, even after a hundred generations,
present no greater distinctions from their ancestors than they do from their contemporaries.
On this we have tangible points of comparison to start from; for the bones of the Egyptian
mummies in one instance, and old collections of butterflies and illustrated works on entomology
in another instance, cover quite as wide an interval as we have assumed above. The
cultivation of the silkworm in China would cover thousands instead of hundreds of generations,
if any ancient pictures were extant, which is quite possible among a nation like the
Chinese; and this may at some future time throw a light on the subject. The fact of individual
variation is looked upon as a mainstay of the hypothesis of modified descent; but
how are we to define the force sufficiently powerful to nullify variation over even the short
period we have quoted, and to control it to such an extent that each species is able to retain
its identity, and that not a single instance of varieties of known descent becoming settled in
possession of different diagnostic characteristics can be adduced. The ground taken that
these individual differences multiplying in the direction most advantageous to the species
ought to produce permanent alterations, is merely a deduction. The facts before us, though
excessively limited in extent, tend to show that they do not produce permanent alterations.
I t is inferred that species are so produced, because i t is found possible to conceive that they
might be so produced, and that if so produced the process would fit in with a grand and
comprehensive theory: but it is impossible to avoid the admission that, as far as we know
from actual proof positive, species are not produced by this process or by any o t h e r ; and it is
yet unproven that new species are in the course of formation at all, though of this there are
many possible indications.
W i t h regard to the belief in the immutability of species, t h e strongest objection seems to
be the apparently conclusive evidence furnished by geology of the successive rise, prevalence,
and ultimate extinction of many genera. This objection even in the present crude state of
geological research seems formidable, and must become more so from day to day as fresh dis