PHYSIOLOGICAL SELECTION.
So far we have been concerned with the principle of Isolation in
general. We have now to consider that form of isolation which arises in
consequence of mutual infertility between the members of any group of
organisms and those of all other similarly isolated groups occupying
simultaneously the same area.
* * * * *
Against the view that natural selection is a sufficient explanation of
the origin of species, there are two fatal difficulties: one, the
contrast between natural species and domesticated varieties in respect
of cross-sterility; the other, the fact that natural selection cannot
possibly give rise to polytypic as distinguished from monotypic
evolution. Now it is my belief that the theory of physiological
selection fully meets both these difficulties. Indeed I hold this to be
undeniable in a formal or logical sense: the only question is as to the
evidence which can be adduced for the theory in a practical or
biological sense. Therefore in this chapter, where the theory has first
of all to be stated, I shall restrict the exposition as much as possible
to the former, leaving for subsequent consideration the biological
side.
The following is a brief outline sketch of this theory[15].
[15] _See Nineteenth Century_, January, 1887, pp. 61, 62.
Of all parts of those variable objects which we call organisms, the most
variable is the reproductive system; and the variations may carry with
them functional changes, which may be either in the direction of
increased or of diminished fertility. Consequently variations in the way
of greater or less fertility frequently take place, both in plants and
animals; and probably, if we had adequate means of observing this point,
we should find that there is no one variation more common. But of course
where infertility arises--whether as a result of changed conditions of
life, or, as we say, spontaneously--it immediately becomes extinguished,
seeing that the individuals which it affects are less able (if able at
all) to propagate and to hand on the variation. If, however, the
variant, while showing some degree of infertility with the parent form,
continues to be as fertile as before when mated with similar variants,
under these circumstances there is no reason why such differential
fertility should not be perpetuated.
Stated in another form this suggestion enables us to regard many, if not
most, species as the records of variations in the reproductive systems
of their ancestors. When variations of a non-useful kind occur in any of
the other systems or parts of organisms, they are, as a rule,
immediately extinguished by intercrossing. But whenever they arise in
the reproductive system in the way here suggested, they tend to be
preserved as new natural varieties, or incipient species. At first the
difference would only be in respect of the reproductive systems; but
eventually, on account of independent variation, other differences would
supervene, and the variety would take rank as a true species.
Now we must remember that physiological isolation is not like those
other forms of isolation (e.g. geographical) which depend for their
occurrence on accidents of the environment, and which may therefore take
place suddenly in a full degree of completeness throughout a large
section of a species. Physiological isolation depends upon distinctive
characters belonging to organisms themselves; and it would be opposed to
the whole theory of descent with progressive modification to imagine
that absolute sterility usually arises, in a single generation between
two sections of a perfectly fertile species. Therefore evolutionists
must believe that in most, if not in all cases--could we trace the
history, say of any two species, which having sprung from a single
parent stock on a common area, are now absolutely sterile with one
another--we should find that this mutual sterility had been itself a
product of gradual evolution. Starting from complete fertility within
the limits of a single parent species, the infertility between
derivative or divergent species, _at whatever stage in their evolution
this began to occur_, must usually at first have been well-nigh
imperceptible, and thenceforth have proceeded to increase stage by
stage.
But, if it be true that physiological isolation between genetically
allied groups must usually itself have been the product of a gradual
evolution; and if, when fully evolved, it constitutes a condition of the
first importance to any further differentiation of these groups (by
preventing fusion again into one group, more or less resembling the
original parent form), do we not perceive at least a strong probability
that in the lower stages of its evolution such mutual infertility must
have acted as a segregating influence between the diverging types, in a
degree proportional to its own development? The importance of mutual
sterility as a condition to divergent evolution is not denied, _when
this sterility is already present in an absolute degree_; and we have
just seen that, before it can have attained to this absolute degree _it
must presumably, and as a rule, itself have been the subject of a
gradual development_. Does it not therefore become, on merely antecedent
grounds, in a high degree probable, that from the moment of its
inception this isolating agency must have played the part of a
segregating cause, in a degree proportional to that of its completeness
as a physiological character?
Whoever answers this question in the affirmative will have gone most of
the way towards accepting, on merely antecedent grounds, the theory of
physiological selection. And therefore it is that I have begun this
statement of the theory by introducing it upon these grounds, thereby
hoping to show how extremely simple--how almost self-evident--is the
theory which it will now be my endeavour to substantiate. I may here add
that the theory was foreshadowed by Mr. Belt in 1874[16], clearly
enunciated in its main features by Mr. Catchpool in 1884[17], and very
fully thought out by Mr. Gulick during a period of about fifteen years,
although he did not publish until a year after the appearance of my own
paper in 1886[18].
[16] _Nicaragua_, p. 207.
[17] _Nature_, vol. xxxi. p. 4.
[18] _Zool. Journal, Lin. Soc._, vol. xix. pp. 337-411 (1886); and
for Mr. Gulick's papers, _ibid._, vol. xx. pp. 189-274 (1887), vol.
xxiii. pp. 312-380 (1889). Mr. Gulick has recently drawn my
attention, in a private letter, to the fact that as early as 1872 a
paper of his was read at the British Association, bearing the title
_Diversity of Evolution under one set of External Conditions_, and
that here the principle of physiological segregation is stated.
Although it does not appear that Mr. Gulick then appreciated the
great importance of this principle, it entitles him to claim
priority.
I must next proceed to state some of the leading features of
physiological selection in further detail.
It has already been shown that Darwin clearly perceived that the very
general occurrence of some degree of infertility between allied species
cannot possibly be attributed to the _direct_ agency of natural
selection. His explanation was that the slight structural modifications
entailed by the transformation of one specific type into another, so
react upon the highly delicate reproductive system of the changing type
as to render it in some degree infertile with its parent type. Now the
theory of physiological selection begins by traversing this view. It
does not, however, deny that in _some_ cases the morphological may be
the prior change; but it strenuously denies that this must be so in
_all_ cases. Indeed, according to my statement in 1886, the theory
inclines to the view that, _as a rule_, the physiological change is
prior. At the same time, the theory, as I have always stated it,
maintains that it is immaterial whether, "in the majority of instances,"
the physiological change has been prior to the morphological, or vice
versa; since in either case the physiological change will equally make
for divergence of character.
To show this clearly the best way will be to consider the two cases
separately, taking first that in which the physiological change has
priority. In this case our theory regards any morphological changes
which afterwards supervene as due to the independent variability which
will sooner or later arise under the physiological isolation thus
secured. But to whatever causes the subsequent morphological changes may
be due, the point to notice is that they are as a general rule,
consequent upon the physiological change. For in whatever _degree_ such
infertility arises between two sections of a species occupying the same
area, in that _degree_ is their interbreeding prevented, and, therefore,
opportunity is given for a subsequent divergence of type, whether by the
influence of independent variability alone, or also by that of natural
selection, as now acting more or less independently on each of the
partially separated groups. In short, all that was said in the foregoing
chapters with respect to isolation in general, here applies to
physiological isolation in particular; and by supposing such isolation
to have been the prior change, we can as well understand the subsequent
appearance of morphological divergence on continuous areas, as in other
forms of isolation we can understand such divergence on discontinuous
areas, seeing that even a moderate degree of cross-infertility may be as
effectual for purposes of isolation as a high mountain-chain, or a
thousand miles of ocean.
Here, then, are two sharply-defined theories to explain the very general
fact of there being some greater or less degree of cross-infertility
between allied species. The older, and hitherto current theory,
supposes the cross-infertility to be but an _accident_ of specific
divergence, which, therefore, has nothing to do with _causing_ the
divergence. The newer theory, on the other hand, supposes the
cross-infertility to have often been a necessary _condition_ to the
divergence having begun at all. Let us now consider which theory has
most evidence in its favour.
First of all we have to notice the very general occurrence of the fact
in question. For when we include the infertility of hybrids, as well as
first crosses, the occurrence of some degree of infertility between
allied species is so usual that Mr. Wallace recommends experiments to
ascertain whether careful observation might not prove, even of species
which hybridize, "that such species, when crossed with their near
allies, do always produce offspring which are more or less sterile
_inter se_[19]." This seems going too far, but nevertheless it is the
testimony of a highly competent naturalist to the very general
occurrence of an association between the morphological differentiation
of species and the fact of a physiological isolation. Now I regard it as
little short of self-evident that this general association between
mutual infertility and innumerable secondary, or relatively variable
morphological distinctions, is due to the former having been an original
and a necessary condition to the occurrence of the latter, in cases
where intercrossing has not been otherwise prevented.
[19] _Darwinism_, p. 169.
The importance of physiological isolation, _when once fully developed_,
cannot be denied, for it is evident that if such isolation could be
suddenly destroyed between two allied species occupying a common area,
they would sooner or later become fused into a common type--supposing,
of course, no other form of isolation to be present. The necessity then
for this physiological form of isolation in _maintaining_ a specific
differentiation which has been already _attained_ cannot be disputed.
Yet it has been regarded as "Darwinian heresy" to suggest that it can
have been of any important service _during the process of attainment_,
or while the specific differentiation is being advanced, and this
notwithstanding that the physiological change must presumably have
developed _pari passu_ with the morphological, and notwithstanding that
in countless cases the former is associated with every conceivable
variety of the latter.
Again, why should the physiological change be thus associated with
_every conceivable variety_ of morphological change? Throughout the
length and breadth of both vegetable and animal kingdoms we find this
association, in the great majority of cases, where new species arise.
Therefore, on the supposition that in all such cases the physiological
change has been adventitiously induced by the morphological changes, we
have to face an apparently unanswerable question--Why should the
reproductive mechanism of all organic beings have been thus arranged, as
it were, to change in immediate response to the very slightest
alteration in the complex harmony of "somatic" processes, which now more
than ever is recognized as exercising so comparatively little influence
on the _hereditary_ endowments of this mechanism? Consider the
difference between a worm and the bird that is eating it, an oak tree
and the gall-insect that is piercing it: are we to suppose that in all
cases, no matter how greatly the types differ, they must agree in this,
that when any parts of these complex structures change, ever so
slightly, the reproductive system is almost certain to be adventitiously
affected, yet always thus affected in the same peculiar way?
If it be answered that the reproductive system is known to be very
sensitive to slight changes in the external conditions of life, the
answer proves too much. For though this is true, yet our opponents must
acknowledge that the reproductive system is not so sensitive, _in this
particular respect_, as their interpretation of the origin of specific
infertility requires. The proof of this point is overwhelming, for there
is the evidence from the entire range of our domesticated productions,
both vegetable and animal. Here the amount of structural change, which
has been slowly accumulated by artificial selection, is often much
greater in amount, and incomparably more rapid, than that which has been
induced between allied species by natural selection; and yet there is
scarcely any indication of the reproductive system having been affected
in the particular way that our opponents' theory requires. There are
many instances of its having been affected in sundry other ways
(chiefly, however, without any accompanying morphological change); but
among all the thousands of our more or less enormously modified
artificial types, there is scarcely one instance of such a peculiar
sexual relation between the modified descendants of a common type as so
usually obtains between allied species in nature. Yet in all other
respects evolutionists are bound to believe that the process of
modification has been in both cases strictly analogous. Why then this
conspicuous difference with respect to the reproductive system?
The answer is simple. It has never been the object of breeders or of
horticulturists to select variations in the direction of
cross-infertility, for the swamping effects of intercrossing are much
more easily and rapidly prevented by artificial isolation. Consequently,
although they have been able to modify natural types in so many
directions and in such high degrees with regard to _morphology_, there
has been no accompanying physiological modification of the kind
required. But in nature there is no such thing as artificial, i.e.
intentional, isolation. Consequently, on common areas it must usually
happen that those changes of morphology which are associated with
cross-infertility are the only ones which can arise. Hence the very
remarkable contrast between our domesticated varieties and natural
species with regard to cross-infertility is just what the present theory
would expect, or, indeed, require. But on any other theory it has
hitherto remained inexplicable.
In particular, the contrast in question has constituted one of the main
difficulties with which the theory of natural selection has hitherto had
to contend, not only in the popular mind, but also in the judgement of
naturalists, including the joint-authors of the theory themselves. Thus
Darwin says:--
The fertility of varieties is, with reference to my theory, of
equal importance with the sterility of species, for it seems to
make a broad and clear distinction between varieties and
species[20].
[20] _Origin of Species_, p. 136.
And Mr. Wallace says:--
One of the greatest, or perhaps we may say the greatest, of all the
difficulties in the way of accepting the theory of natural
selection as a complete explanation of the origin of species, has
been the remarkable difference between varieties and species in
respect of fertility when crossed[21].
[21] _Darwinism_, p. 152.
Now, in view of this conspicuous contrast, Darwin suggested that species
in a state of nature "will have been exposed during long periods of time
to more uniform conditions than have domesticated varieties, and [that]
this may well make a wide difference in the result." Now we have to
remember that species, living and extinct, are numbered by millions, and
represent every variety of type, constitution, and habits; is it
probable, then, that this one peculiarity of the reproductive system
should be due, in so many cases, to some merely incidental effect
produced on that system by uniform conditions of life? Again, _ex
hypothesi_, at the time when a variety is first forming, the influence
exercised by uniform conditions of life (whatever in different cases
this may happen to be) cannot be present as regards that variety: yet
this is just the time when its infertility with the parent (or allied)
form is most likely to have arisen; for it is just then that the nascent
variety would otherwise have been most liable to extinction by free
intercrossing--even supposing that in the presence of such intercrossing
the variety could ever have come into existence at all.
Mr. Wallace meets the difficulty by arguing that sterility between
allied species may have been brought about by the direct influence of
natural selection. But, as previously remarked, this view is expressly
opposed to that of Darwin, who held that Wallace's contention is
erroneous.
It will be seen, then, that both Darwin, and Wallace, fully recognize
the necessity of finding some explanation of the infertility of allied
species, over and above the mere reaction of morphological
differentiation on the physiology of the reproductive system, and they
both agree in suggesting additional causes, though they entirely
disagree as to what these causes are. Now, the theory of physiological
selection likewise suggests an additional cause--or, rather, a new
explanation--and one which is surely the most probable. For what is to
be explained? The very general association of a certain physiological
peculiarity with that amount of morphological change which distinguishes
species from species, of whatever kind the change may be, and in
whatever family of the animal or vegetable kingdom it may occur. Well,
the theory of physiological selection explains this very general
association by the simple supposition that, at least in a large number
of cases, it was the physiological peculiarity which first of all led to
the morphological divergence, by interposing the bar of sterility
between two sections of a previously uniform species; and by thus
isolating the two sections one from another, started each upon a
subsequently independent course of divergent evolution.
Or, to put it in another way, if the occurrence of this physiological
peculiarity has been often the only possible means of isolating two
sections of a species occupying a common area, and thus giving rise to a
divergence of specific type (as obviously _must_ have been the case
wherever there was an absence of any other form of isolation), it is
nothing less than a necessary consequence that many allied species
should now present the physiological peculiarity in question. Thus the
association between the physiological peculiarity and the morphological
divergence is explained by the simple hypothesis, that the former has
acted as a necessary condition to the occurrence of the latter. In the
absence of other forms of isolation, the morphological divergence could
not have taken place at all, had not the physiological peculiarity
arisen; and hence it is that we now meet with so many cases where such
divergence is associated with this peculiarity.
* * * * *
So far we have been considering the physiological change as historically
the prior one. Here, at first sight, it may seem that the segregative
power of physiological selection must end; for it may well seem
impossible that the physiological change can ever be necessary for the
divergence of morphological varieties into true species in cases where
it has _not_ been the prior change, but has only set in after
morphological changes have proceeded far enough to have already
constituted definite varieties. A little thought, however, will show
that physiological selection is quite as potent a condition to the
differentiation of species when it occurs after varietal divergence has
begun, as it is when it occurs before the divergence--and hence that it
really makes no difference to the theory of physiological selection
whether, in particular cases, the cross-infertility arises before or
after any structural or other modifications with which it is
associated.
For the theory does not assert that all varieties have been due to
physiological selection. There are doubtless many other causes of the
origin of varieties besides cross-infertility with parent forms; but, as
a general rule, it does not appear that they are by themselves capable
of carrying divergence beyond a merely varietal stage. In order to carry
divergence to the stage of producing _species_, it appears to be a
general condition that, sooner or later, cross-infertility should
arise--seeing that, when varieties do succeed in becoming species, we
almost invariably find that, as a matter of fact, cross-infertility has
arisen. Hence, if cross-infertility has thus usually been a necessary
condition to a varietal divergence becoming specific, it can make no
material difference when the incipient infertility arose.
It may be asked, however, whether I suppose that, when the physiological
change is subsequent, it is directly _caused_ by change of structure,
size, colour, &c., or that it arises, so to speak, accidentally, from
other causes which may have affected the sexual system in the required
way. To this question I may briefly reply, that, looking to the absence
of any influence exercised on the reproductive systems of our
domesticated plants and animals by the great and varied changes which so
many of these forms present, it would seem that among natural varieties
such closely analogous changes are presumably not the usual causes of
the physiological change, even where the latter are subsequent to the
former. Nevertheless, I do not deny that in some of these cases changes
of structure, size, colour, &c., may be the causes of the physiological
change by reacting on the sexual system in the required way. But in
such cases free intercrossing will have prevented the perpetuation of
any morphological changes, save those which have the power of so
reacting on the reproductive system as to produce the physiological
change, and thus to protect themselves against the full and adverse
power of free intercrossing. We know that slight or initial changes of
structure, colour, &c., frequently occur as varieties, and yet that on
common areas very few of these varieties become distinct species: free
intercrossing prevents any such further divergence of character. But if
in the course of many such abortive attempts, as it were, to produce a
new species, nature happens to hit upon a structural or a colour
variation which is capable of reacting on the sexual system in the
particular way required, then this variation will be enabled to protect
itself against free intercrossing in proportion to its own development.
Or, in other words, the more it develops as a morphological change, the
more will it increase the physiological change; while the more the
physiological change is thus increased, the more will it in turn promote
the morphological. By such action and reaction the development of each
furthers the development of the other, till from an almost imperceptible
variety, apparently quite fertile with its parent form, there arises a
distinct species absolutely sterile with its parent form. In such cases,
therefore, it is still the physiological conditions which have
_selected_ the particular morphological changes capable of so reacting
on the reproductive system as to produce cross-infertility, and thus to
protect themselves against the destructive power of free intercrossing.
So to speak, free intercrossing is always on the watch to level down
any changes which natural selection, or any other cause of varietal
divergence, may attempt to produce; and therefore, in order to
produce--or to increase--such divergence in the absence of any other
form of isolation, natural selection must hit upon such changes of
structure, form, or colour, as are so correlated with the reproductive
system as to create the physiological isolation that is required.
To show how the principle of selective fertility may be combined with
what apparently is the most improbable form of isolation for this
purpose--the geographical--I quote the following suggestion made by
Professor Lloyd Morgan in his _Animal Life and Intelligence_:--
Suppose two divergent local varieties were to arise in adjacent
areas, and were subsequently (by stress of competition or by
geographical changes) driven together into a single area.... If
their unions be fertile, the isolation will be annulled by
intercrossing--the two varieties will form one mean or average
variety. But if the unions be infertile, the isolation will be
preserved, and the two varieties will continue separate. Suppose
now, and the supposition is by no means an improbable one, that
this has taken place again and again in the evolution of species;
then it is clear that those varietal forms which had continued to
be fertile together would be swamped by intercrossing; while those
varietal forms which had become infertile would remain isolated.
Hence, in the long run, isolated forms occupying a common area
would be infertile, (p. 107.)
If then cross-sterility may thus arise even in association with
geographical isolation, may it not also arise in its absence? And may it
not thus give rise to the differentiation of varieties on account of
this physiological isolation alone?
Only two further points need be mentioned to make this statement of
physiological selection as complete as the present _résumé_ of its main
principles requires.
The first is, that, as Mr. Wallace remarks, "every species has come into
existence coincident both in space and time with a pre-existing and
closely allied species." I regard this as important evidence that
physiological selection is one of the natural causes concerned. For the
general fact implied is that every species has come into existence on an
area occupied by its parent type, and therefore under circumstances
which render it imperative that intercrossing with that type should be
prevented. In the case of monotypic evolution by natural selection
alone, intercrossing with the parent type is prevented through the
gradual extinction of that type by successive generations of the
developing type. But in the case of polytypic evolution, intercrossing
with the parent type can only be prevented by some form of isolation
other than natural selection; and here it is evident that
cross-infertility with the parent type must be as efficient to that end
as any other form of isolation that can be imagined. Consequently we
might almost have expected beforehand that in a large proportional
number of cases cross-infertility should have been the means employed.
And the fact that this is actually the case so far corroborates the only
theory which is able to explain it.
The second point is this.
It appears to be comparatively rare for any cause of specific divergence
to prove effectual on common areas, unless it sooner or later becomes
associated with some degree of cross-infertility. But through this
association, the segregating influence of both the causes concerned is,
as Mr. Gulick has shown, greatly increased. For instance, if the
segregating influence of some degree of cross-infertility be associated
with that of any other form of isolation, then, not only will the two
segregating influences be added, but multiplied together. And thus, by
their mutual action and reaction, divergent evolution is promoted at a
rapidly increasing rate.
I will now summarize the main points of the theory of physiological
isolation in a categorical form.
1. If no other form of isolation be present, specific divergence can
only take place when some degree of cross-infertility has previously
arisen between two or more sections of a species.
2. When such cross-infertility has arisen it may cause specific
divergence, either (_a_) by allowing independent variability in each of
the physiologically isolated groups; (_b_) by becoming associated with
any other cause of differentiation already operating; or (_c_) by both
these means combined.
3. As some degree of cross-infertility generally obtains between allied
species, we are justified in concluding that this has been the most
frequent--or, at any rate, the most effective--kind of isolation where
the origin of species is concerned; and therefore the kind with which,
in the case of species-formation, natural selection, or any other cause
of specific divergence, has been most usually associated.
4. Where varietal divergence has begun in the absence of
cross-infertility, such divergence seems, as a general rule, to have
been incapable of attaining to a specific value.
5. Therefore, in the vast majority of such cases, it must have been
those varietal changes of structure, size, colour, &c., which happened
to have afterwards been assisted by the reproductive change that were on
this account _selected_ as successful candidates for specific
differentiation.
6. It follows, that it makes no difference to the general theory of
physiological selection in what proportion of cases the physiological
change has been the initial change; for, whether prior or subsequent to
the varietal changes with which it becomes associated, its presence has
been equally important as a condition to specific divergence.
7. When physiological isolation becomes associated with natural
selection, or any other form of homogamy, the segregative power of both
is augmented. Moreover, so great is the augmentation that even very
moderate degrees of physiological isolation--themselves capable of
effecting little or nothing--become very powerful when associated with
moderate degrees of any other kind of homogamy, and vice versa.
8. The theory of physiological selection effectually explains the
divergent evolution of specific types and the cross-infertility of such
types when evolved.
* * * * *
To prevent, if possible, the continuance of certain misunderstandings
with regard to my original statement of the new theory, let me here
disclaim some views which have been assigned to me. They are:
1. That the theory of physiological selection is opposed to the theory
of natural selection. Far from this being so, it is--at all events in my
own opinion--a very important aid to it, in preventing free
intercrossing on a common area, and thus allowing divergent evolution to
occur within that area.
2. That, in advancing the theory of physiological selection as "an
additional suggestion on the origin of species," I wish to represent it
as being the originating cause of _all_ species. What I hold is, that
all species must have owed their origin to _isolation_, in some form or
other; but that as physiological selection is only one among many other
forms of isolation (including natural selection), and as it can only act
on common areas, a large number of species must have been formed without
its aid.
3. That I imagine physiological varieties always to arise
"sporadically," or as merely individual "sports" of the reproductive
system. On the contrary, I expressly stated that this is _not_ the way
in which I suppose the "physiological variation" to arise, when giving
origin to a new species; but that it arises, whenever it is effectual,
as a "collective variation" affecting a number of individuals
simultaneously, and therefore characterizing "a whole race, or strain."
4. That I suppose physiological selection always to act alone. This I
have never supposed. The essential point is, not that the physiological
isolation is unassociated with other forms of isolation, but that unless
associated with some degree of physiological isolation, no one of the
other forms is capable of originating species on common areas with any
approach to frequency. This proposition is the essence of the new
theory, and I take it to be proved, not only by general deductive
reasoning which shows that it _must_ be so, but also by the fact of an
otherwise inexplicable association between specific divergence on common
areas and some more or less considerable degree of mutual infertility.