different groups of animals and plants, we should obtain a body of facts
of a character now sadly wanting, and without which it is hopeless to
expect to arrive at a complete solution of this difficult problem. There
are, however, some other aspects of the question that need to be
considered, and some theoretical views which require to be carefully
examined, having done which we shall be in a condition to state the
general conclusions to which the facts and reasonings at our command
seem to point.
_Sterility due to changed Conditions and usually correlated with other
Characters, especially with Colour._
The evidence already adduced as to the extreme susceptibility of the
reproductive system, and the curious irregularity with which infertility
or sterility appears in the crosses between some varieties or species
while quite absent in those between others, seem to indicate that
sterility is a characteristic which has a constant tendency to appear,
either by itself or in correlation with other characters. It is known to
be especially liable to occur under changed conditions of life; and, as
such change is usually the starting-point and cause of the development
of new species, we have already found a reason why it should so often
appear when species become fully differentiated.
In almost all the cases of infertility or sterility between varieties or
species, we have some external differences with which it is correlated;
and though these differences are sometimes slight, and the amount of the
infertility is not always, or even usually, proportionate to the
external difference between the two forms crossed, we must believe that
there is some connection between the two classes of facts. This is
especially the case as regards colour; and Mr. Darwin has collected a
body of facts which go far to prove that colour, instead of being an
altogether trifling and unimportant character, as was supposed by the
older naturalists, is really one of great significance, since it is
undoubtedly often correlated with important constitutional differences.
Now colour is one of the characters that most usually distinguishes
closely allied species; and when we hear that the most closely allied
species of plants are infertile together, while those more remote are
fertile, the meaning usually is that the former differ chiefly in the
_colour_ of their flowers, while the latter differ in the form of the
flowers or foliage, in habit, or in other structural characters.
It is therefore a most curious and suggestive fact, that in all the
recorded cases, in which a decided infertility occurs between varieties
of the same species, those varieties are distinguished by a difference
of colour. The infertile varieties of Verbascum were white and yellow
flowered respectively; the infertile varieties of maize were red and
yellow seeded; while the infertile pimpernels were the red and the blue
flowered varieties. So, the differently coloured varieties of
hollyhocks, though grown close together, each reproduce their own colour
from seed, showing that they are not capable of freely intercrossing.
Yet Mr. Darwin assures us that the agency of bees is necessary to carry
the pollen from one plant to another, because in each flower the pollen
is shed before the stigma is ready to receive it. We have here,
therefore, either almost complete sterility between varieties of
different colours, or a prepotent effect of pollen from a flower of the
same colour, bringing about the same result.
Similar phenomena have not been recorded among animals; but this is not
to be wondered at when we consider that most of our pure and valued
domestic breeds are characterised by definite colours which constitute
one of their distinctive marks, and they are, therefore, seldom crossed
with these of another colour; and even when they are so crossed, no
notice would be taken of any slight diminution of fertility, since this
is liable to occur from many causes. We have also reason to believe that
fertility has been increased by long domestication, in addition to the
fact of the original stocks being exceptionally fertile; and no
experiments have been made on the differently coloured varieties of wild
animals. There are, however, a number of very curious facts showing that
colour in animals, as in plants, is often correlated with constitutional
differences of a remarkable kind, and as these have a close relation to
the subject we are discussing, a brief summary of them will be here
given.
_Correlation of Colour with Constitutional Peculiarities._
The correlation of a white colour and blue eyes in male cats with
deafness, and of the tortoise-shell marking with the female sex of the
same animal, are two well-known but most extraordinary cases. Equally
remarkable is the fact, communicated to Darwin by Mr. Tegetmeier, that
white, yellow, pale blue, or dun pigeons, of all breeds, have the young
birds born naked, while in all other colours they are well covered with
down. Here we have a case in which colour seems of more physiological
importance than all the varied structural differences between the
varieties and breeds of pigeons. In Virginia there is a plant called the
paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours
their bones pink, and causes the hoofs of all but the black varieties to
drop off; so that black pigs only can be kept in the district.[58]
Buckwheat in flower is also said to be injurious to white pigs but not
to black. In the Tarentino, black sheep are not injured by eating the
Hypericum crispum - a species of St. John's-wort - which kills white
sheep. White terriers suffer most from distemper; white chickens from
the gapes. White-haired horses or cattle are subject to cutaneous
diseases from which the dark coloured are free; while, both in Thuringia
and the West Indies, it has been noticed that white or pale coloured
cattle are much more troubled by flies than are those which are brown or
black. The same law even extends to insects, for it is found that
silkworms which produce white cocoons resist the fungus disease much
better than do those which produce yellow cocoons.[59] Among plants, we
have in North America green and yellow-fruited plums not affected by a
disease that attacked the purple-fruited varieties. Yellow-fleshed
peaches suffer more from disease than white-fleshed kinds. In Mauritius,
white sugar-canes were attacked by a disease from which the red canes
were free. White onions and verbenas are most liable to mildew; and
red-flowered hyacinths were more injured by the cold during a severe
winter in Holland than any other kinds.[60]
These curious and inexplicable correlations of colour with
constitutional peculiarities, both in animals and plants, render it
probable that the correlation of colour with infertility, which has been
detected in several cases in plants, may also extend to animals in a
state of nature; and if so, the fact is of the highest importance as
throwing light on the origin of the infertility of many allied species.
This will be better understood after considering the facts which will be
now described.
_The Isolation of Varieties by Selective Association._
In the last chapter I have shown that the importance of geographical
isolation for the formation of new species by natural selection has been
greatly exaggerated, because the very change of conditions, which is
the initial power in starting such new forms, leads also to a local or
stational segregation of the forms acted upon. But there is also a very
powerful cause of isolation in the mental nature - the likes and
dislikes - of animals; and to this is probably due the fact of the
comparative rarity of hybrids in a state of nature. The differently
coloured herds of cattle in the Falkland Islands, each of which keeps
separate, have been already mentioned; and it may be added, that the
mouse-coloured variety seem to have already developed a physiological
peculiarity in breeding a month earlier than the others. Similar facts
occur, however, among our domestic animals and are well known to
breeders. Professor Low, one of the greatest authorities on our
domesticated animals, says: "The female of the dog, when not under
restraint, makes selection of her mate, the mastiff selecting the
mastiff, the terrier the terrier, and so on." And again: "The Merino
sheep and Heath sheep of Scotland, if two flocks are mixed together,
each will breed with its own variety." Mr. Darwin has collected many
facts illustrating this point. One of the chief pigeon-fanciers in
England informed him that, if free to choose, each breed would prefer
pairing with its own kind. Among the wild horses in Paraguay those of
the same colour and size associate together; while in Circassia there
are three races of horses which have received special names, and which,
when living a free life, almost always refuse to mingle and cross, and
will even attack one another. On one of the Faroe Islands, not more than
half a mile in diameter, the half-wild native black sheep do not readily
mix with imported white sheep. In the Forest of Dean, and in the New
Forest, the dark and pale coloured herds of fallow deer have never been
known to mingle; and even the curious Ancon sheep of quite modern origin
have been observed to keep together, separating themselves from the rest
of the flock when put into enclosures with other sheep. The same rule
applies to birds, for Darwin was informed by the Rev. W.D. Fox that his
flocks of white and Chinese geese kept distinct.[61]
This constant preference of animals for their like, even in the case of
slightly different varieties of the same species, is evidently a fact
of great importance in considering the origin of species by natural
selection, since it shows us that, so soon as a slight differentiation
of form or colour has been effected, isolation will at once arise by the
selective association of the animals themselves; and thus the great
stumbling-block of "the swamping effects of intercrossing," which has
been so prominently brought forward by many naturalists, will be
completely obviated.
If now we combine with this fact the correlation of colour with
important constitutional peculiarities, and, in some cases, with
infertility; and consider, further, the curious parallelism that has
been shown to exist between the effects of changed conditions and the
intercrossing of varieties in producing either an increase or a decrease
of fertility, we shall have obtained, at all events, a starting-point
for the production of that infertility which is so characteristic a
feature of distinct species when intercrossed. All we need, now, is some
means of increasing or accumulating this initial tendency; and to a
discussion of this problem we will therefore address ourselves.
_The Influence of Natural Selection upon Sterility and Fertility._
It will occur to many persons that, as the infertility or sterility of
incipient species would be useful to them when occupying the same or
adjacent areas, by neutralising the effects of intercrossing, this
infertility might have been increased by the action of natural
selection; and this will be thought the more probable if we admit, as we
have seen reason to do, that variations in fertility occur, perhaps as
frequently as other variations. Mr. Darwin tells us that, at one time,
this appeared to him probable, but he found the problem to be one of
extreme complexity; and he was also influenced against the view by many
considerations which seemed to render such an origin of the sterility or
infertility of species when intercrossed very improbable. The fact that
species which occupy distinct areas, and which nowhere come in contact
with each other, are often sterile when crossed, is one of the
difficulties; but this may perhaps be overcome by the consideration
that, though now isolated, they may, and often must, have been in
contact at their origination. More important is the objection that
natural selection could not possibly have produced the difference that
often occurs between reciprocal crosses, one of these being sometimes
fertile, while the other is sterile. The extremely different amounts of
infertility or sterility between different species of the same genus,
the infertility often bearing no proportion to the difference between
the species crossed, is also an important objection. But none of these
objections would have much weight if it could be clearly shown that
natural selection _is_ able to increase the infertility variations of
incipient species, as it is certainly able to increase and develop all
useful variations of form, structure, instincts, or habits. Ample causes
of infertility have been shown to exist, in the nature of the organism
and the laws of correlation; the agency of natural selection is only
needed to accumulate the effects produced by these causes, and to render
their final results more uniform and more in accordance with the facts
that exist.
About twenty years ago I had much correspondence and discussion with Mr.
Darwin on this question. I then believed that I was able to demonstrate
the action of natural selection in accumulating infertility; but I could
not convince him, owing to the extreme complexity of the process under
the conditions which he thought most probable. I have recently returned
to the question; and, with the fuller knowledge of the facts of
variation we now possess, I think it may be shown that natural selection
_is_, in some probable cases at all events, able to accumulate
variations in infertility between incipient species.
The simplest case to consider, will be that in which two forms or
varieties of a species, occupying an extensive area, are in process of
adaptation to somewhat different modes of life within the same area. If
these two forms freely intercross with each other, and produce mongrel
offspring which are quite fertile _inter se_, then the further
differentiation of the forms into two distinct species will be retarded,
or perhaps entirely prevented; for the offspring of the crossed unions
will be, perhaps, more vigorous on account of the cross, although less
perfectly adapted to the conditions of existence than either of the pure
breeds; and this would certainly establish a powerful antagonistic
influence to the further differentiation of the two forms.
Now, let us suppose that a partial sterility of the hybrids between the
two forms arises, in correlation with the different modes of life and
the slight external or internal peculiarities that exist between them,
both of which we have seen to be real causes of infertility. The result
will be that, even if the hybrids between the two forms are still freely
produced, these hybrids will not themselves increase so rapidly as the
two pure forms; and as these latter are, by the terms of the problem,
better suited to their conditions of life than are the hybrids between
them, they will not only increase more rapidly, but will also tend to
supplant the hybrids altogether whenever the struggle for existence
becomes exceptionally severe. Thus, the more complete the sterility of
the hybrids the more rapidly will they die out and leave the two parent
forms pure. Hence it will follow that, if there is greater infertility
between the two forms in one part of the area than the other, these
forms will be kept more pure wherever this greater infertility prevails,
will therefore have an advantage at each recurring period of severe
struggle for existence, and will thus ultimately supplant the less
infertile or completely fertile forms that may exist in other portions
of the area. It thus appears that, in such a case as here supposed,
natural selection would preserve those portions of the two breeds which
were most infertile with each other, or whose hybrid offspring were most
infertile; and would, therefore, if variations in fertility continued to
arise, tend to increase that infertility. It must particularly be noted
that this effect would result, not by the preservation of the infertile
variations on account of their infertility, but by the inferiority of
the hybrid offspring, both as being fewer in numbers, less able to
continue their race, and less adapted to the conditions of existence
than either of the pure forms. It is this inferiority of the hybrid
offspring that is the essential point; and as the number of these
hybrids will be permanently less where the infertility is greatest,
therefore those portions of the two forms in which infertility is
greatest will have the advantage, and will ultimately survive in the
struggle for existence.
The differentiation of the two forms into distinct species, with the
increase of infertility between them, would be greatly assisted by two
other important factors in the problem. It has already been shown that,
with each modification of form and habits, and especially with
modifications of colour, there arises a disinclination of the two forms
to pair together; and this would produce an amount of isolation which
would greatly assist the specialisation of the forms in adaptation to
their different conditions of life. Again, evidence has been adduced
that change of conditions or of mode of life is a potent cause of
disturbance of the reproductive system, and, consequently, of
infertility. We may therefore assume that, as the two forms adopted more
and more different modes of life, and perhaps acquired also decided
peculiarities of form and coloration, the infertility between them would
increase or become more general; and as we have seen that every such
increase of infertility would give that portion of the species in which
it arose an advantage over the remaining portions in which the two
varieties were more fertile together, all this induced infertility would
maintain itself, and still further increase the general infertility
between the two forms of the species.
It follows, then, that specialisation to separate conditions of life,
differentiation of external characters, disinclination to cross-unions,
and the infertility of the hybrid produce of these unions, would all
proceed _pari passu_, and would ultimately lead to the production of two
distinct forms having all the characteristics, physiological as well as
structural, of true species.
In the case now discussed it has been supposed, that some amount of
general infertility might arise in correlation with the different modes
of life of two varieties or incipient species. A considerable body of
facts already adduced renders it probable that this _is_ the mode in
which any widespread infertility would arise; and, if so, it has been
shown that, by the influence of natural selection and the known laws
which affect varieties, the infertility would be gradually increased.
But, if we suppose the infertility to arise sporadically within the two
forms, and to affect only a small proportion of the individuals in any
area, it will be difficult, if not impossible, to show that such
infertility would have any tendency to increase, or would produce any
but a prejudicial effect. If, for example, five per cent of each form
thus varied so as to be infertile with the other form, the result would
be hardly perceptible, because the individuals which formed cross-unions
and produced hybrids would constitute a very small portion of the whole
species; and the hybrid offspring, being at a disadvantage in the
struggle for existence and being themselves infertile, would soon die
out, while the much more numerous fertile portion of the two forms would
increase rapidly, and furnish a sufficient number of pure-bred offspring
of each form to take the place of the somewhat inferior hybrids between
them whenever the struggle for existence became severe. We must suppose
that the normal fertile forms would transmit their fertility to their
progeny, and the few infertile forms their infertility; but the latter
would necessarily lose half their proper increase by the sterility of
their hybrid offspring whenever they crossed with the other form, and
when they bred with their own form the tendency to sterility would die
out except in the very minute proportion of the five per cent
(one-twentieth) that chance would lead to pair together. Under these
circumstances the incipient sterility between the two forms would
rapidly be eliminated, and could never rise much above the numbers which
were produced by sporadic variation each year.
It was, probably, by a consideration of some such case as this that Mr.
Darwin came to the conclusion that infertility arising between incipient
species could not be increased by natural selection; and this is the
more likely, as he was always disposed to minimise both the frequency
and the amount even of structural variations.
We have yet to notice another mode of action of natural selection in
favouring and perpetuating any infertility that may arise between two
incipient species. If several distinct species are undergoing
modification at the same time and in the same area, to adapt them to
some new conditions that have arisen there, then any species in which
the structural or colour differences that have arisen between it and its
varieties or close allies were correlated with infertility of the
crosses between them, would have an advantage over the corresponding
varieties of other species in which there was no such physiological
peculiarity. Thus, incipient species which were infertile together would
have an advantage over other incipient species which were fertile, and,
whenever the struggle for existence became severe, would prevail over
them and take their place. Such infertility, being correlated with
constitutional or structural differences, would probably, as already
suggested, go on increasing as these differences increased; and thus, by
the time the new species became fully differentiated from its parent
form (or brother variety) the infertility might have become as well
marked as we usually find it to be between distinct species.
This discussion has led us to some conclusions of the greatest
importance as bearing on the difficult problem of the cause of the
sterility of the hybrids between distinct species. Accepting, as highly
probable, the fact of variations in fertility occurring in correlation
with variations in habits, colour, or structure, we see, that so long as
such variations occurred only sporadically, and affected but a small
proportion of the individuals in any area, the infertility could not be
increased by natural selection, but would tend to die out almost as fast
as it was produced. If, however, it was so closely correlated with
physical variations or diverse modes of life as to affect, even in a
small degree, a considerable proportion of the individuals of the two
forms in definite areas, it would be preserved by natural selection, and
the portion of the varying species thus affected would increase at the
expense of those portions which were more fertile when crossed. Each
further variation towards infertility between the two forms would be
again preserved, and thus the incipient infertility of the hybrid
offspring might be increased till it became so great as almost to amount
to sterility. Yet further, we have seen that if several competing
species in the same area were being simultaneously modified, those
between whose varieties infertility arose would have an advantage over
those whose varieties remained fertile _inter se_, and would ultimately
supplant them.
The preceding argument, it will be seen, depends entirely upon the
assumption that some amount of infertility characterises the distinct
varieties which are in process of differentiation into species; and it
may be objected that of such infertility there is no proof. This is
admitted; but it is urged that facts have been adduced which render such
infertility probable, at least in some cases, and this is all that is
required. It is by no means necessary that _all_ varieties should
exhibit incipient infertility, but only, some varieties; for we know
that, of the innumerable varieties that occur but few become developed
into distinct species, and it may be that the absence of infertility, to
obviate the effects of intercrossing, is one of the usual causes of
their failure. All I have attempted to show is, that _when_ incipient
infertility does occur in correlation with other varietal differences,
that infertility can be, and in fact must be, increased by natural
selection; and this, it appears to me, is a decided step in advance in
the solution of the problem.[62]
_Physiological Selection._
Another form of infertility has been suggested by Professor G.J. Romanes