fact, I was so convinced that not even a stripe of colour appears from what
would commonly be called an accident, that I was led solely from the
occurrence of the face-stripes on this hybrid from the ass and hemionus, to
ask Colonel Poole whether such face-stripes ever occur in the eminently
striped Kattywar breed of horses, and was, as we have seen, answered in the
affirmative.
What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation, striped
on the legs like a zebra, or striped on the shoulders like an ass. In the
horse we see this tendency strong whenever a dun tint appears--a tint which
approaches to that of the general colouring of the other species of the
genus. The appearance of the stripes is not accompanied by any change of
form or by any other new character. We see this tendency to become striped
most strongly displayed in hybrids from between several of the most
distinct species. Now observe the case of the several breeds of pigeons:
they are descended from a pigeon (including two or three sub-species or
geographical races) of a bluish colour, with certain bars and other marks;
and when any breed assumes by simple variation a bluish tint, these bars
and other marks invariably reappear; but without any other change of form
or character. When the oldest and truest breeds of various colours are
crossed, we see a strong tendency for the blue tint and bars and marks to
reappear in the mongrels. I have stated that the most probable hypothesis
to account for the reappearance of very ancient characters, is--that there
is a tendency in the young of each successive generation to produce the
long-lost character, and that this tendency, from unknown causes, sometimes
prevails. And we have just seen that in several species of the horse-genus
the stripes are either plainer or appear more commonly in the young than in
the old. Call the breeds of pigeons, some of which have bred true for
centuries, species; and how exactly parallel is the case with that of the
species of the horse-genus! For myself, I venture confidently to look back
thousands on thousands of generations, and I see an animal striped like a
zebra, but perhaps otherwise very differently constructed, the common
parent of our domestic horse, whether or not it be descended from one or
more wild stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created, will, I
presume, assert that each species has been created with a tendency to vary,
both under nature and under domestication, in this particular manner, so as
often to become striped like other species of the genus; and that each has
been created with a strong tendency, when crossed with species inhabiting
distant quarters of the world, to produce hybrids resembling in their
stripes, not their own parents, but other species of the genus. To admit
this view is, as it seems to me, to reject a real for an unreal, or at
least for an unknown, cause. It makes the works of God a mere mockery and
deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created in
stone so as to mock the shells now living on the sea-shore.
Summary. -- Our ignorance of the laws of variation is profound. Not in one
case out of a hundred can we pretend to assign any reason why this or that
part differs, more or less, from the same part in the parents. But
whenever we have the means of instituting a comparison, the same laws
appear to have acted in producing the lesser differences between varieties
of the same species, and the greater differences between species of the
same genus. The external conditions of life, as climate and food, &c.,
seem to have induced some slight modifications. Habit in producing
constitutional differences, and use in strengthening, and disuse in
weakening and diminishing organs, seem to have been more potent in their
effects. Homologous parts tend to vary in the same way, and homologous
parts tend to cohere. Modifications in hard parts and in external parts
sometimes affect softer and internal parts. When one part is largely
developed, perhaps it tends to draw nourishment from the adjoining parts;
and every part of the structure which can be saved without detriment to the
individual, will be saved. Changes of structure at an early age will
generally affect parts subsequently developed; and there are very many
other correlations of growth, the nature of which we are utterly unable to
understand. Multiple parts are variable in number and in structure,
perhaps arising from such parts not having been closely specialised to any
particular function, so that their modifications have not been closely
checked by natural selection. It is probably from this same cause that
organic beings low in the scale of nature are more variable than those
which have their whole organisation more specialised, and are higher in the
scale. Rudimentary organs, from being useless, will be disregarded by
natural selection, and hence probably are variable. Specific
characters--that is, the characters which have come to differ since the
several species of the same genus branched off from a common parent--are
more variable than generic characters, or those which have long been
inherited, and have not differed within this same period. In these remarks
we have referred to special parts or organs being still variable, because
they have recently varied and thus come to differ; but we have also seen in
the second Chapter that the same principle applies to the whole individual;
for in a district where many species of any genus are found--that is, where
there has been much former variation and differentiation, or where the
manufactory of new specific forms has been actively at work--there, on an
average, we now find most varieties or incipient species. Secondary sexual
characters are highly variable, and such characters differ much in the
species of the same group. Variability in the same parts of the
organisation has generally been taken advantage of in giving secondary
sexual differences to the sexes of the same species, and specific
differences to the several species of the same genus. Any part or organ
developed to an extraordinary size or in an extraordinary manner, in
comparison with the same part or organ in the allied species, must have
gone through an extraordinary amount of modification since the genus arose;
and thus we can understand why it should often still be variable in a much
higher degree than other parts; for variation is a long-continued and slow
process, and natural selection will in such cases not as yet have had time
to overcome the tendency to further variability and to reversion to a less
modified state. But when a species with any extraordinarily-developed
organ has become the parent of many modified descendants--which on my view
must be a very slow process, requiring a long lapse of time--in this case,
natural selection may readily have succeeded in giving a fixed character to
the organ, in however extraordinary a manner it may be developed. Species
inheriting nearly the same constitution from a common parent and exposed to
similar influences will naturally tend to present analogous variations, and
these same species may occasionally revert to some of the characters of
their ancient progenitors. Although new and important modifications may
not arise from reversion and analogous variation, such modifications will
add to the beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference in the offspring from
their parents--and a cause for each must exist--it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face of
this earth are enabled to struggle with each other, and the best adapted to
survive.
Chapter VI
Difficulties on Theory
Difficulties on the theory of descent with modification -- Transitions --
Absence or rarity of transitional varieties -- Transitions in habits of
life -- Diversified habits in the same species -- Species with habits
widely different from those of their allies -- Organs of extreme perfection
-- Means of transition -- Cases of difficulty -- Natura non facit saltum --
Organs of small importance -- Organs not in all cases absolutely perfect --
The law of Unity of Type and of the Conditions of Existence embraced by the
theory of Natural Selection.
Long before having arrived at this part of my work, a crowd of difficulties
will have occurred to the reader. Some of them are so grave that to this
day I can never reflect on them without being staggered; but, to the best
of my judgment, the greater number are only apparent, and those that are
real are not, I think, fatal to my theory.
These difficulties and objections may be classed under the following
heads:- Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
animal with wholly different habits? Can we believe that natural selection
could produce, on the one hand, organs of trifling importance, such as the
tail of a giraffe, which serves as a fly-flapper, and, on the other hand,
organs of such wonderful structure, as the eye, of which we hardly as yet
fully understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to so marvellous an instinct as that which leads the bee
to make cells, which have practically anticipated the discoveries of
profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?
The two first heads shall be here discussed--Instinct and Hybridism in
separate chapters.
On the absence or rarity of transitional varieties. -- As natural selection
acts solely by the preservation of profitable modifications, each new form
will tend in a fully-stocked country to take the place of, and finally to
exterminate, its own less improved parent or other less-favoured forms with
which it comes into competition. Thus extinction and natural selection
will, as we have seen, go hand in hand. Hence, if we look at each species
as descended from some other unknown form, both the parent and all the
transitional varieties will generally have been exterminated by the very
process of formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of the
earth? It will be much more convenient to discuss this question in the
chapter on the Imperfection of the geological record; and I will here only
state that I believe the answer mainly lies in the record being
incomparably less perfect than is generally supposed; the imperfection of
the record being chiefly due to organic beings not inhabiting profound
depths of the sea, and to their remains being embedded and preserved to a
future age only in masses of sediment sufficiently thick and extensive to
withstand an enormous amount of future degradation; and such fossiliferous
masses can be accumulated only where much sediment is deposited on the
shallow bed of the sea, whilst it slowly subsides. These contingencies
will concur only rarely, and after enormously long intervals. Whilst the
bed of the sea is stationary or is rising, or when very little sediment is
being deposited, there will be blanks in our geological history. The crust
of the earth is a vast museum; but the natural collections have been made
only at intervals of time immensely remote.
But it may be urged that when several closely-allied species inhabit the
same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling nearly the same
place in the natural economy of the land. These representative species
often meet and interlock; and as the one becomes rarer and rarer, the other
becomes more and more frequent, till the one replaces the other. But if we
compare these species where they intermingle, they are generally as
absolutely distinct from each other in every detail of structure as are
specimens taken from the metropolis inhabited by each. By my theory these
allied species have descended from a common parent; and during the process
of modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent and all
the transitional varieties between its past and present states. Hence we
ought not to expect at the present time to meet with numerous transitional
varieties in each region, though they must have existed there, and may be
embedded there in a fossil condition. But in the intermediate region,
having intermediate conditions of life, why do we not now find
closely-linking intermediate varieties? This difficulty for a long time
quite confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an
area is now continuous, that it has been continuous during a long period.
Geology would lead us to believe that almost every continent has been
broken up into islands even during the later tertiary periods; and in such
islands distinct species might have been separately formed without the
possibility of intermediate varieties existing in the intermediate zones.
By changes in the f
orm of the land and of climate, marine areas now
continuous must often have existed within recent times in a far less
continuous and uniform condition than at present. But I will pass over
this way of escaping from the difficulty; for I believe that many perfectly
defined species have been formed on strictly continuous areas; though I do
not doubt that the formerly broken condition of areas now continuous has
played an important part in the formation of new species, more especially
with freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a common
alpine species disappears. The same fact has been noticed by Forbes in
sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important elements
of distribution, these facts ought to cause surprise, as climate and height
or depth graduate away insensibly. But when we bear in mind that almost
every species, even in its metropolis, would increase immensely in numbers,
were it not for other competing species; that nearly all either prey on or
serve as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings, we must see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing physical
conditions, but in large part on the presence of other species, on which it
depends, or by which it is destroyed, or with which it comes into
competition; and as these species are already defined objects (however they
may have become so), not blending one into another by insensible
gradations, the range of any one species, depending as it does on the range
of others, will tend to be sharply defined. Moreover, each species on the
confines of its range, where it exists in lessened numbers, will, during
fluctuations in the number of its enemies or of its prey, or in the
seasons, be extremely liable to utter extermination; and thus its
geographical range will come to be still more sharply defined.
If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each has a
wide range, with a comparatively narrow neutral territory between them, in
which they become rather suddenly rarer and rarer; then, as varieties do
not essentially differ from species, the same rule will probably apply to
both; and if we in imagination adapt a varying species to a very large
area, we shall have to adapt two varieties to two large areas, and a third
variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and
lesser area; and practically, as far as I can make out, this rule holds
good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between
well-marked varieties in the genus Balanus. And it would appear from
information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
generally when varieties intermediate between two other forms occur, they
are much rarer numerically than the forms which they connect. Now, if we
may trust these facts and inferences, and therefore conclude that varieties
linking two other varieties together have generally existed in lesser
numbers than the forms which they connect, then, I think, we can understand