STERILITY OF HYBRIDS. CONCLUDING REMARKS.
When we consider the bearing of recent discoveries on those
comprehensive schemes of evolution with which we were formerly
satisfied, we find that certain details of the process are more easy
to imagine. We readily now understand how varieties once formed,
can persist, but at the same time difficulties hitherto faced with
complacency become formidable in the light of the new knowledge. So
generally is this admitted by those familiar with modern genetic
research that most are rightly inclined to postpone the discussion. The
premisses, indeed, on which such a discussion must be based are almost
wholly wanting.
The difficulties to which I chiefly refer are not those created by the
phenomena of adaptation, though they are serious enough. In treating
of that subject I have felt obliged to express scepticism as to the
validity of nearly all the new evidence for the transmission of acquired
characters. At the present time the utmost we are bound to accept is the
proof that (1) in some parthenogenetic forms variations, or perhaps we
may say malformations, produced in response to special conditions, recur
in one or perhaps two generations asexually produced after removal to
other conditions. (2) That violent maltreatment may in rare instances
so affect the germ-cells contained in the parents as to cause the
individuals resulting from the fertilisation of those cells to exhibit
an arrest of development similar to that which their parents underwent.
I do not doubt that evidence of this type will be greatly extended. As
a contribution to genetic physiology these facts are very important
and interesting, but I cannot think that any one, on reflexion, will
feel encouraged by such indications to revive old beliefs in the direct
origin of adaptations.
In these respects we are simply left where we were. The force of
objections based upon the existence of adaptative mechanisms is
no greater than it has always been. On the contrary the fact that
variations can now so generally be recognized as definite is some
alleviation of the difficulty. We can moreover disabuse ourselves of
the notion that for all characters which are definite or fixed, some
utilitarian rationale may be presumed. Upon that point the study of
variation has provided a perfectly clear answer.
In frankly recognizing that the fixity of characters in general need
not connote usefulness to their possessors we deliver ourselves of a
distracting pre-occupation and prepare our minds for an investigation
of the properties of living organisms in the same spirit as that
in which the chemist and the physicist examine the properties of
unorganized materials. The creature persists not merely by virtue of its
characteristics but in spite of them, and the fact of its persistence
proves no more than that on the whole the balance of its properties
leaves something in its favour.
It may be noted by the way that the fact that the structures of living
things are on the whole adaptative was not always obvious. Though to
naturalists of this generation it is a truism, we have only to turn to
Buffon to find that in his philosophy of nature it played no essential
part. The passage in which Buffon describes what he regards as the
forlorn and degraded condition of the Woodpecker is well known. We have
come to think of the Woodpecker as a capital example of adaptation to
the mode of life; but Buffon after enumerating the hard features of
the bird's existence, forced to earn its living by piercing the bark
of trees in an attitude of perpetual constraint, remarks[1] "Tel est
l'instinct étroit et grossier d'un oiseau borné a une vie triste et
chétive. Il a reçu de la Nature des organes et des instrumens appropriés
a cette destinée _ou plutôt il tient cette destinée même des organes
avec lesquels il est né_" (my italics). His reflexions on the Stilt
(_Himantopus_) read even more strangely to us, accustomed as we are to
see in the prodigious length and thinness of the shanks and in the other
features of its organisation palpable adaptations to a wading life. For
Buffon, however, this curious bird seemed a poor, neglected production,
extravagant in its disproportions, one of the misfits of creation,
left as a shadow in the picture composed of nature's more successful
efforts.[2] This theme he develops at some length, being evidently well
pleased with the idea.
Our way of regarding these things is doubtless sounder and more fruitful
than Buffon's, but it is well to remember that what seems so obvious to
us looked quite differently to other excellent observers; and stupid
as it may have been to have overlooked plain examples of adaptation,
it is a far worse mistake to see adaptation everywhere. I do not seek
to minimise the real and permanent difficulty which the existence of
adaptations creates, but by the suggestion that all normal specific
differences are adaptational that difficulty was quite gratuitously
increased.
In these respects it may be claimed that progress has been made, even if
that progress seem outwardly of small account.
But all constructive theories of evolution have been built on the
understanding that what we know of the relation of varieties to species
justifies the assumption that the one phenomenon is a phase of the
other, and that each species arises or has arisen from another species
either by one or several genetic steps. In the varieties we have
accustomed ourselves to think that we see those steps. We still know
little enough of the mode of occurrence of variation, but we do begin to
know something, and if we ask ourselves whether our knowledge, such as
it is, conforms at all readily with our former expectations, we cannot
with any confidence assert that it does. Among the plants and animals
genetically investigated are many illustrations of very striking and
distinct varieties. Many of these might readily enough be accepted as
species by even the most exacting systematists, and not a few have
been so treated in classification; but when we have examined their
relationship to each other we feel not merely that they are not species
in any strict sense but that the distinctions they present cannot be
regarded as stages in the direction of specific difference. Complete
fertility of the results of inter-crossing is and I think must rightly
be regarded as inconsistent with actual specific difference; and of
variations leading to that consequence no clear indication has yet been
found. As an example of possible exceptions mention should perhaps be
made of the case of a giant form of _Primula sinensis_ investigated by
Keeble.[3] It arose from a "Star" Primula of normal size, and though
fertile with its own pollen all attempts to fertilise it with the pollen
of other forms failed. Miss Pellew, who did these fertilisations,
tells me that very extensive trials were made, and repeated in several
seasons. Ultimately two plants were raised from it fertilised with a
plant of the strain from which it sprang, and these proved sterile.
In the light of modern experience the significance of such isolated
instances is doubtful.
All the strains known as "Giants" are, as Messrs. Sutton have always
found, more or less sterile, and their sterility is presumably due to
some negative defect.
In regard to the fertility of Primula species there are several
paradoxes. For example the long-styled varieties, apart from giants, are
fertile with their own pollen, and for many years short-styled plants
have not been used in most strains. Auriculas and Polyanthuses, on the
contrary, are generally if not always bred from short-styled plants,
as the florists have decided that the long-styled are inadmissible.
Mr. R. P. Gregory tells me that, though most strains of _P. sinensis_
give seed enough when only long-styled plants are used, he finds
nevertheless that when a "legitimate" union is made the amount of seed
usually increases much as Darwin observed. Darwin's statement that
plants of "illegitimate" origin are less fertile than the "legitimately"
raised plants is also in general confirmed by his experience. To
this rule there were some marked exceptions in strains derived from
_long_-styled plants, which though illegitimate showed a high degree
of fertility, but illegitimate unions between _short_-styled plants
always produced comparatively sterile offspring. I have no records of
the behavior of Auriculas and Polyanthuses. It would be interesting to
know whether among them pure strains of short-styled plants (dominants)
have appeared, and, if so, how their fertility is affected. Without
much more critical data I suppose no one would nowadays be inclined
to follow Darwin in instituting a comparison between the sterility
of hybrids and that of illegitimately raised plants of heterostyle
species.[4] It is even difficult to imagine any essential resemblance
between these two phenomena, nor has evidence ever been produced to
show that illegitimately raised plants have bad pollen grains, which is
the usual symptom of sterility in hybrid plants and the consequence,
as we believe, of failure of some essential division in the process of
maturation.
The difficulty that we have no knowledge of the contemporary origin of
forms, from a common stock, which when crossed together give a sterile
product, is one of the objections constantly and prominently adduced
from the time of the first promulgation of evolutionary ideas. In the
light of recent work the objection has gathered strength. Why, if
we are able to produce instances of variation colourably simulating
specific difference in almost all other respects, do we never find
an original appearance of this most widely spread of all specific
characteristics? No doubt all breeders know that sterile animals
and plants occasionally appear in their cultures, but it is more in
accordance with probability that the sterility in these sporadic
instances should be regarded as due to defect than that it should be
thought comparable with that of the sterile hybrids. For their sterility
must, by all analogy with results elsewhere seen, be attributed not
to the absence of something, but to the presence and operation of
complementary factors leading to the production of inhibition of
division; and consistently with that interpretation, we find that when
from a partially sterile hybrid comparatively fertile offspring can be
raised, their comparative fertility continues in the posterity generally
if not always without diminution. The distinction between these several
kinds of sterility was of course not understood in Darwin's time. The
comparison, for example, which he instituted[5] between the sterility
of "contabescent" anthers and that of hybrids no longer holds, for at
least in those cases in which the nature of contabescent anthers have
been genetically investigated (Sweet Pea, _Tropaeolum_) they proved
to be a simple recessive character. Nor can we now easily suppose that
the attempt there made by Darwin to suggest resemblance between the
sterility produced by unnatural conditions and that of hybrids has any
physiological justification.
In regarding the power to produce a sterile or partially sterile hybrid
as a distinction in kind, of a nature other than those which we perceive
among our varieties, I am aware that I am laying stress on an impression
which may hereafter prove false. The distinction nevertheless is so
striking and so continually before the eyes of a practical breeder that
he can scarcely avoid the inference that when he meets a considerable
degree of sterility in a cross-bred he is dealing with something
belonging to a distinct category, and not merely a varietal feature of
an exceptional kind.
Besides the sterility of hybrids appeal has often been made to the
phenomenon of incompatibility, in its several stages of completeness,
as distinguishing species. No one doubts that incompatibility may
arise from a variety of causes of most diverse degrees of importance,
but though sometimes referred to as an extreme case of interspecific
sterility, it is really a very different matter. In regard to one phase
of this incompatibility, that associated with self-sterility, some
progress has been made, and we are not wholly without experimental
evidence of its being within the range of contemporary variation.
Given the outline of Mendelian teaching as to gametic differentiation
and the classification of individuals in a mixed population, it
seemed highly probable that what we call self-sterility must mean
that the species really consisted of _classes_, some of which are
capable of interbreeding with others while others are not. According
to the received account every individual, though incapable of
fertilising itself, was supposed to be able both to fertilise and to
be fertilised by any other individual. This notion has always seemed
to me a self-evident absurdity, for it would imply that there can be
as many categories as individuals. Such experiments, however, as I
made did certainly give results consistent with that belief. I first
tried Cinerarias, which are usually self-sterile, but I found no
incompatible pairs of plants. Whether I was deceived by the consequences
of apogamy, or whether the pollen of certain plants may belong to more
than one class I do not know. The results were confused in various
ways. Usually the self-fertilised plants set little or nothing, and
cross-fertilised they set fully with such uniformity that the few
failures could plausibly be attributed to mistakes in manipulation
or to other extraneous causes. Later de Vries announced[6] (without
giving particulars) that he had proved the existence of such classes in
_Linaria vulgaris_; but on making experiments with that species I again
got no positive results, and I came to the conclusion that in spite of
inherent improbability the conventional belief must be substantially
true. At last, however, the work of Correns, lately published,[7] does
definitely show that in one species, _Cardamine pratensis_, classes of
individuals exist such that individuals of the same class are incapable
of fertilising themselves or each other, but fertilisation made between
the classes is usually completely effective. Many complications were
encountered and some contradictory evidence is recorded, but the general
bearing of the results was positive and indubitable.
We know far too little of this phenomenon as yet to be able to
understand its significance, but I suppose we may anticipate with some
confidence that it will be found to be a manifestation of dissimilarity
between the male and female gametes of the same individual, comparable
with that first seen in the Stocks (_Matthiola_) which throw doubles--a
state of things in all likelihood to be found widely spread among
hermaphrodite organisms. Whether the incompatibility between species
is to be associated with that of the self-steriles also cannot be
positively asserted, though it seems not unreasonable to expect that
such an association will be discovered.
The case of the apple and the pear is an impressive illustration of
this possibility. The two species are of course exceedingly alike in
all outward respects, but nevertheless the pollen of each is entirely
without effect on the other. Presumably we should interpret this fact
as meaning not so much that the apple and the pear are in reality
very wide apart, but rather that either, each is lacking in one of
two complementary elements, or that each possesses a factor with an
inhibitory effect. Their incompatibility may well be of the same nature
as that of the classes in _Cardamine pratensis_.
Returning now to the problem of inter-specific sterility; we note,
as I have said, the absence of contemporary evidence that variation
can confer on a variety the power to form a sterile hybrid with the
parent species. The considerations based on this want of evidence have
for a long while been familiar to all who have discussed evolutionary
theories, and it is worth observing the exact reason why the difficulty
strikes us now with a new and special force. In pre-Mendelian times
all that was known was that some forms could freely interbreed without
diminution of fertility in the product, while others could not. But now
we find that, by virtue of segregation, from one and the same pair of
parents, or even, in the case of hermaphrodites, from one and the same
individual, offspring commonly arises showing among themselves exactly
such differences as distinguish species--and very good species too. This
we see happening again and again. But to forms capable of arising as
brethren in one family the title species has never been meant to apply,
and if we are going to use the term in application to fraternal groups
we must definitely recognise that by "specific" difference is to be
understood simply _difference_, without any immediate or even ulterior
physiological limitation whatever. Naturally, therefore, we begin to
think of the appearance of sterility in crosses as something apart, and
as a manifestation which distinguishes certain kinds of unions in a very
special way.
I am perfectly aware that there are gradations in the sterility of
hybrids as in every other characteristic upon which it has been proposed
to base specific definitions; but, as also so often happens in the
matter of defining intergrading categories, the difficulty in practice
is not often such as to lead to actual ambiguity. I am speaking of
course of those examples which are amenable to genetic experiment.
As to the rest there is complete and permanent uncertainty. But the
experience of the practical breeder does, I think, on the whole, support
the contention to which systematists have so steadily clung under all
the assaults of evolutionary philosophers, that, though we cannot
strictly define species, they yet have properties which varieties have
not, and that the distinction is not merely a matter of degree.
The first step is to discover the nature of the factors which by their
complementary action inhibit the critical divisions and so cause
the sterility of the hybrid. Thus expressed, we see the problem of
inter-specific sterility in its right place; and the question why we
do not now find contemporary instances of varieties lately arisen in
domestication, which when crossed back with their parents, or with their
coderivatives, can produce sterile products, is perceived to be only a
special case of a problem which in its more general form is that of the
origin of new and additional factors.
For the requisite evidence no comprehensive search has been made, but
perhaps it will yet be found. All that we can say at the present time
is that the incidence both of hybrid sterility, and of incompatibility
also, is most capricious; and provided that two forms have such features
in common that a cross between them seems not altogether out of the
question, no one can predict without experiment whether such a cross
is feasible, and if feasible whether the product will be fertile, or
sterile more or less completely. For instance, though probably all
the British and some Foreign Finches (Fringillidae) have been crossed
together, and some of these crosses, as for instance, the various
Canary-mules have been made in thousands, I believe no quite clear
example of a fertile hybrid can be produced. Many species of Anatidae
cross readily and produce fertile hybrids: others give results uniformly
sterile. Though most of the Equidae can be crossed and some of the
hybrids are among the commonest of domesticated animals there is no
certain record of a fertile mule. Among the Canidae the dogs, wolves and
jackals all give fertile hybrids, but there is no clearly authenticated
instance of a cross between any of these forms and the European fox.
In spite of their close anatomical resemblance it is doubtful if the
rabbit and the hare have ever interbred. Many of the wild species of
_Bos_ have been crossed and recrossed both with each other and with
many domesticated races, but I understand that no cross with the Indian
buffalo (_Bos bubalus_) has yet been successful even in producing a
live calf.[8] In the genus _Primula_ many hybrids are known and several
of them occur in nature, but hitherto no certain hybrid between _P.
sinensis_ and any other species has been made, in spite of repeated
attempts.
In _Nicotiana_ many--doubtless all--the various forms of _N. tabacum_
can be crossed together without diminution of fertility, though some
are very distinct in appearance, but crosses between _tabacum_ and
_sylvestris_ are highly sterile (in my experience totally sterile[9]),
though the distinctions between them are not to outward observation
nearly so great as those which can be found between the various races of
_Primula sinensis_.
Recently some remarkable experiments bearing closely on these questions
have been published by F. Rosen.[10] They concern the forms of _Erophila
(Draba) verna_, celebrated in the history of evolutionary theory as the
plants especially chosen by Alexis Jordan for the exposition of his
views on these subjects.
The "species" contains a profusion of forms dissimilar in many
structural characters, such as the size and shape of leaves, flowers,
fruits, etc. Of these forms many grow in association. Jordan found, on
experiment, that each, to the number of some two hundred, bred true, and
that therefore, the conventional assumption that polymorphism of this
kind must mean great contemporary variability had no foundation in fact.
So far indeed is the evidence from favouring the belief that such forms
are in any way transitional or indeterminate, that, as is well known,
Jordan used it with every plausibility to support the doctrine of the
fixity of species. To certain aspects of Jordan's work we will return
later in this chapter, but the matter is in the present connection
of especial interest for the reason that Rosen has lately found by
experiment that some of these presumably very closely allied forms,
crossed together, gave hybrids more or less sterile. In the case of the
offspring of one pair of forms only (_E. cochleata_ and _stricta_) was
the fertility undiminished, and the various degrees of sterility found
in the other crosses ranged up to the extreme infertility of the hybrids
between _E. stricta_ × _elata_. From this cross ten plants were bred.
Of these the four strongest were chosen to breed from, but two of the
four proved totally sterile; one had only bad seeds; and from the fourth
a single seedling was raised which in its turn proved to be sterile.
From the less sterile hybrids F_{2} families were raised, with the
usual experience that in this and subsequent generations the sterility
diminished among extracted forms, new and true-breeding types with
complete fertility being thus derived from the original cross.[11]
The production of sterility as a consequence of crossing plants so
nearly approaching each other as these _Erophila_ "species" do is
not a little interesting, and the fact well exemplifies the futility
of the various attempts to frame general expressions as to specific
properties or behaviour. Commenting on his results Rosen argues that
the polymorphic group commonly called by systematists _Erophila (Draba)
verna_ may now be regarded as having arisen by crossing, as did his own
types mentioned above. The question, however, _what_ species were the
original progenitors of the group cannot be answered. Rosen considers
that no form which he knows satisfies the requirements, and that
it or they must be supposed to be lost. This conclusion will recall
the similar problem raised by the _Oenothera_ mutants (Chap. V); and
unsatisfactory as it may be to have recourse to such hypotheses we
must remember the possibility that as a consequence of hybridisation,
subsequent segregation and recombination of factors, species may
have thus actually, as we may say, exploded, and left nothing but a
polymorphic group of miscellaneous types to represent them in posterity.
If this way of regarding the phenomena be a true one, the sterility
now seen when some of the group are re-crossed, becomes analogous to
that "reversion or crossing" which we now so well understand to be
a consequence of the recombination of characters separated at some
previous point in the history of descent. In the partial sterility of
the contemporary hybrid we see this character reappearing, formed now
as it was on the occasion of the original cross, by the meeting of
complementary factors.
Another case that may be mentioned in this connection is that of the
crosses between various culinary peas (_Pisum sativum_) and a peculiar
form found by Mr. Arthur Sutton growing ostensibly in a wild state in
Palestine. This Palestine Pea is low growing, rarely reaching 18 inches.
It is in general appearance like a small and poorly grown field pea.
The stems are thin and rather hard. The most obvious differences which
distinguish this from other field peas are the marked serration of the
stipules, and the development of pith in the pods. Such pith is often
present in the pods of peas more or less, but in the Palestines it is
so strongly developed as almost to form a lomentum. Curiously enough,
though the flowers are purple much as those of ordinary field peas,
there is no coloured spot in the axils. On the other hand, the stems
have coloured stripes running up from the axils. Though this plant
differs so little from domesticated peas, all crosses with them either
failed, or produced hybrids quite or almost quite sterile. This was Mr.
Sutton's experience, and on repeating the experiments with material
kindly given by him I found the same result.[12]
In a large series of crosses some seeds died or gave rise to feeble
plants. Of the plants which lived, few gave any seed. The seed, however,
that was obtained from F_{1} plants grew well enough, and the F_{2}
plants proved, as often in such cases, fertile. In these, indeed, no
sign of sterility was noticeable. The experiment is being repeated in
various ways, for, as the genetic behaviour of peas is comparatively
well known, the subject is an exceptionally favourable one for these
investigations.
Such an example shows the confusion produced the moment we attempt to
harmonize conceptions of specific difference with results attained by
experimental methods. It has been usual to regard the field pea (_P.
arvense_) as a species distinct from the edible pea (_P. sativum_).
De Candolle and others regard the field pea as derived from a form
wild in Italy, but the origin of the edible pea is considered to be
unknown. From breeding experiments we find no sterility whatever in the
crosses between the various _arvense_ and _sativum_ types, nor in the
crosses made between them and several other peculiar types from various
countries; whereas this Palestine Pea, which only differs from a small
_arvense_ in what might have been thought trivial characters,[13] either
fails to cross altogether or gives a sterile product, whatever type be
chosen as the other parent.
Examples of this kind have at least the merit that they lead to more
precise delimitations of the problem. We are confronted with two
distinct alternatives.
1. We may apply the term Species promiscuously to all distinct forms.
If we do so it must be clearly understood that we cannot even rule out
the several combinations of "presences and absences" represented by the
various types whether wild or domesticated. For we may feel perfectly
assured that at least all the _arvense_ and all the _sativum_ types yet
subjected to experimental tests are on precisely the same level in this
respect. There is no distinction, logical or physiological, to be drawn
between them. Some contain more factors, and others contain fewer. In
some the re-combinations have been brought about by natural variation or
crossing, while the same consequences in the others have resulted from
man's interference.
2. We may follow the conventions of systematists and distinguish the
outstanding or conspicuous forms such as _arvense_, _quadratum_,
_sativum_ and perhaps a few more as species, and leave the rest
unheeded. If this course is followed it must be clearly understood and
permitted as a piece of pure pragmatism, deliberately adopted for the
convenience of cataloguers and collectors, without regard to any natural
fact or system whatsoever.
But while following either the one plan or the other we shall be still
awaiting the answer, which only genetic experiment can provide, to the
question whether among the various types there are some which differ
from the rest in a peculiar way: whether by having groups of characters
linked together in especially durable combinations, or by possessing
ingredients which cause greater or less disturbance in the processes of
cell-division, and especially in the processes of gametic maturation,
when they are united by fertilisation with complementary ingredients.
Before any but the vaguest ideas regarding the nature and significance
of inter-specific sterility can be formed, a vast amount of detailed
work must be done. Sterility as a result of crossing, as well as
that which is alleged sometimes to arise in consequence of changed
conditions, is at best a negative characteristic, and there are endless
opportunities for mistake and misinterpretation in studying features
of this kind. No one, I suppose, would now feel any great confidence
in most of the data which from time to time are resuscitated for the
purpose of such discussions. Even the best collections of evidence, such
as those given by Darwin in _Forms of Flowers_, cannot be regarded as
critical when judged by present-day standards. Nothing short of the most
familiar acquaintance with the habitual behaviour of individuals, and of
strains kept under constant scrutiny for several years would enable the
experimenter to form reliable judgments as to the value to be attached
to observations of this class.
The admission must, however, be faced that nothing in recent work
materially tends to diminish the surprise which has always been felt
at the absence of sterility in the crosses between co-derivatives. We
should expect such groups of forms to behave like the _Erophila_ types,
and frequently to produce sterile products on crossing. Whatever be
the explanation, the fact remains that such evidence is wanting almost
completely. In spite of all that we know of variability nothing readily
comparable with the power to produce a sterile hybrid on crossing with
a near ally, has yet been observed spontaneously arising, though that
characteristic of specificity is one of the most widely distributed
in nature. It may be that the lacuna in our evidence is due merely to
want of attention to this special aspect of genetic inquiry, and on the
whole that is the most acceptable view which can be proposed. But seeing
that naturalists are more and more driven to believe the domesticated
animals and plants to be poly-phyletic in origin--the descendants, that
is to say, of several wild forms--the difficulty is proportionately
greater than it was formerly, when variation spontaneously occurring was
regarded as a sufficient account of their diversity.
CONCLUDING REMARKS.
The many converging lines of evidence point so clearly to the central
fact of the origin of the forms of life by an evolutionary process
that we are compelled to accept this deduction, but as to almost all
the essential features, whether of cause or mode, by which specific
diversity has become what we perceive it to be, we have to confess an
ignorance nearly total. The transformation of masses of population by
imperceptible steps guided by selection, is, as most of us now see, so
inapplicable to the facts, whether of variation or of specificity, that
we can only marvel both at the want of penetration displayed by the
advocates of such a proposition, and at the forensic skill by which it
was made to appear acceptable even for a time.
In place of this doctrine we have little teaching of a positive kind
to offer. We have direct perception that new forms of life may arise
sporadically, and that they differ from their progenitors quite
sufficiently to pass for species. By the success and maintenance of
such sporadically arising forms, moreover, there is no reasonable doubt
that innumerable strains, whether in isolation or in community with
their co-derivatives, have as a fact arisen, which now pass in the
lists of systematists as species. For an excellent account of typical
illustrations I would refer the reader to the book lately published by
R. E. Lloyd[14] on the rat-population of India. The observations there
recorded are typical of the state of things disclosed whenever the
variations of large numbers of individuals are closely investigated,
whether in domestication or in natural conditions.
Guided by such clues we may get a good way into the problem. We see
the origin of colourable species in abundance. Then, however, doubt
arises whether though these new forms are as good species as many
which are accepted as such by even cautious systematists, there may
not be a stricter physiological sense in which the term species can be
consistently used, which would exclude the whole mass of these _petites
espèces_.
If further we find that we have, with certain somewhat doubtful
exceptions, never seen the contemporary origin of a dominant factor, or
of inter-racial sterility between indubitable co-derivatives, it needs
no elaboration of argument to show that the root of the matter has not
been reached.
Examination of the inter-relations of unquestionably distinct species
nearly allied, such as the two common species of _Lychnis_, leads to
the same disquieting conclusion, and the best suggestion we can make
as to their origin is that _conceivably_ they may have arisen as two
re-combinations of factors brought together by the crossing of parent
species, one or both of which must be supposed to be lost.
All this is, as need hardly be said, an unsatisfying conclusion. To
those permanently engaged in systematics it may well bring despair.
The best course for them is once for all to recognise that whether
or no specific distinction may prove hereafter to have any actual
physiological meaning, it is impossible for the systematist with the
means at his disposal to form a judgment of value in any given case.
Their business is purely that of the cataloguer, and beyond that they
cannot go. They will serve science best by giving names freely and by
describing everything to which their successors may possibly want to
refer, and generally by subdividing their material into as many species
as they can induce any responsible society or journal to publish.
Between Jordan with his 200 odd species for _Erophila_, and Grenier and
Godron with one, there is no hesitation possible. Jordan's view, as he
again and again declares with vehemence, is at least a view of natural
facts, whereas the collective species is a mere abstraction, convenient
indeed for librarians and beginners, but an insidious misrepresentation
of natural truth, perhaps more than any other the source of the
plausible fallacies regarding evolution that have so long obstructed
progress.
Nevertheless though we have been compelled to retreat from the
speculative position to which scientific opinion had rashly advanced,
the prospect of permanent progress is greatly better than it was. With
the development of genetic research clear conceptions have at length
been formed of the kind of knowledge required and of the methods by
which it is to be attained. If we no longer see how varieties give
rise to species, we may feel confident that a minute study of genetic
physiology of varieties and species is the necessary beginning of any
critical perception of their inter-relations. It is little more than a
century since no valid distinction between a mechanical mixture and a
chemical combination could be perceived, and in regard to the forms of
life we may well be in a somewhat similar confusion.
As yet the genetic behaviour of animals and plants has only been
sampled. When the work has been done on a scale so large as to provide
generalisations, we may be in a position to declare whether specific
difference is or is not a physiological reality.
FOOTNOTES:
[1] Buffon, _Hist. Nat._, Oiseaux, 1780, VII, p. 3.
[2] Ibid., VIII, p. 115.
[3] Keeble, _Jour. Gen._, 1912, II, p. 173.
[4] _Animals and Plants_, ed. 1, 1868, II, pp. 180-5.
[5] _Animals and Plants_, ed. 1, 1868, II, p. 165.
[6] _Species and Varieties_, 1905, p. 471.
[7] Correns, _Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch. Naturf.
u. Aertze. Münster i. W._, 1912.
[8] This is a case of a somewhat different order and I mention it partly
for that reason as an illustration of the complexity which such negative
instances may present. The difficulty is that though the buffalo and
the zebu can breed together, the foetus is too large to be born alive.
(See Ackermann _Ber. d. Ver. f. Naturk._, Kassel, 1898, p. 69. Prof. S.
Nathusius, of Halle, who has great experience in crossing Bovidae, tells
me that he has always failed to cross the buffalo with other species.)
[9] In a paper to be published in the Report of the Genetic Conference,
Paris, 1911, Bellair states that he obtained some partially fertile
hybrids in the cross _N. sylvestris_ × _tabacum_. As to the various
degrees of sterility in hybrids between _Nicotiana_ species see Lock, R.
H., _Ann. Roy. Bot. Gardens_. Peradeniya, IV, 1909, p. 195.
[10] _Beitrage zur Biol. der Pflanzen._, X, 1911, p. 379.
[11] One very peculiar feature was observed, namely, that all the new
forms in F_{2} which were bred from came true. As I understand, this
statement applied to five such new types, and they were represented
by 76 individuals in F_{3}, but further details on this point are
desirable. Another curious fact was observed, namely that one of the
F_{1} forms (_cochleata_ × _radiata_) when fertilised by _cochleata_
gave a highly polymorphic family, but fertilised by _radiata_ the
resulting offspring were almost uniform.
[12] I also had a few F_{1} seeds given me by Mr. R. H. Lock.
[13] In a paper about to appear in _Jour. Linn. Soc._ Mr. A. W. Sutton
identifies this Palestine pea as _Pisum humile_ of Boissier and Noé.
[14] Lloyd, R. E., _The Growth of Groups in the Animal Kingdom_, London,
1912.
INDEX OF SUBJECTS
PAGE
Abraxa grossulariata, 105,193
Aceras hircina, local variability, 123
Achatinellidae, local forms of, 133
Acquired characters, inheritance of, 188 et seq.,217,233
Acronycta psi, melanic, 138
Adaptation, problem of, 187,234
Agelaius, local forms, 120
Agrotis, fixed and variable species, 25
Alkaptonuria, 83
Alpine Plants, growing larger, if protected, 183
Alpine Varieties, 165
Alytes obstetricans, Kammerer's experiments on, 199,210
Amblystoma, races of, 230
Amphidasys betularia, melanic form, 136,138
dimorphic larvae, 141
Anodonta, polymorphism of, 130
Antirrhinum, striped, 57
species-hybrids, 99
albinos, 110
Apple, will not cross with pear, 239
Arctia caja, effects of temperature, 192
larval variation in, 231
Arctic varieties, 165
Argynnis paphia and valesina in Italy, 121
Armadillo, polyembryony, 42
Artistic faculty, 89
Arum, rights and lefts, 57
Auriculas, short-styled selected, 236
Axis of symmetry in hand and foot, 48
Axolotl, alleged effect of conditions, 230
Azalea, bud-sports, 55
Bacillus anthracis, unsegmented form, 71
Bacillus prodigiosus, variation in, 213
Bacteria, variation in, 212
Bacterium coli, variation in, 214
Baeolophus, geographical races of, 159
Barley, right and left-handed, 58
Basilarchia, geographical races of, 161
Begonia phyllomaniaca, 50
hybrids, 51
Bizarre Carnation, genetics of, 54
Black, as a variation from red, 148
Blackbird, varying, 150
Black Cock, fixity of, 28
Boarmia repandata, melanic form, 136
rhomboidaria, 137,139
Botrytis susceptibility to, 108
Bovidae, hybrid, 242
Brachydactyly, 89,95
Bradypus, vertebral variation, 68
Bud-sports geometrically irregular, 54-57
Buffalo, attempts to hybridize, 242
Bullfinch, gynandromorph, 45
Bulimus detritus, local variation of, 126
Canary, asymmetrical markings in, 154
Canidae, hybrid, 241
Capsella, 100
Cardamine pratensis, 239
Cat, Polydactylism, 53
Carnation, Picotees and bizarres compared, 54,58
Cataract, hereditary, 89
Certhiola, melanic, 142
Chladni figures, 60
Choloepus, vertebral variation in, 68
local variation in, 119
Cinerarias, self-sterility in, 238
Cistudo, local variation in, 119
Climatic varieties, 164
Coccaceae, variation in, 213
Coenonympha arcania, climatic forms of, 179
satyrion, 180
Coereba, melanic, 142
Colaptes, geographical races, 147 et seq.
chrysoides, 154
Colloids, growth in, 65
Colorado beetles, experiments on, 218
Colour blindness in twins, 44
Continuous variation, possible example of, 173
Coracias, geographical races of, 160
Cotton, genetics of, 98,100
Coupling, 110
Crab, extra claws, 74
Crustacean appendages and Serial Homology, 63
Crystals, analogy with, 78
Cyclopian monsters, artificial, 50
Daphnia, changed by environment, 216
Dasypus, polyembryony, 42
Dianthoecia, fixed and variable species, 25
Disease-resistance, 87
Division, power of,
a fundamental attribute of living things, 38
Genetics of, 46,50
Dogger Bank, large varieties on, 125
Dogs, hybrid, 241
Dominance, nature of, 95
Dominants, origin of new, 88,90,95
Double monsters, 42
Draba, experiments with, 242
Drosophila, 91
Payne's experiments on, 228
Earthworm, regeneration, 77
Elephant, tusk segmented, 38
Entelechy, 80
Environmental treatment, effects of, 188 et seq.
Enzymes and genetic factors, 86
Epilepsy, inheritance of traumatic, 197
Equidae, sterility of hybrid, 241
Erophila, experiments with, 242
species, 249
Exacum, right and left, 57
Euphonia elegantissima, local forms, 120
Eupithecia rectangulata, melanic form, 137
Factors, new, 88
loss of, 96
Factorial representation of varieties, 158,165
Falcons, geographical races, 147
Fasciation, 49
Ferments, Boyle on, 54
Finger-prints of twins, 44
Fixity and Variability in species, 25
Flax, climatic experiments, 197
Fowl, Silky, 84
Leghorn, 85,90
Dominant white, 94
Wyandotte, 97
Rumpless, 46
Foxes, incompatibility with dogs, 241
Free-martin, 44
Fringillidae, sterility of hybrid, 241
Fundulus, cyclopian, 50
Gallus, invariability of wild species, 13
and origin of poultry, 90,97
Genitalia, a basis for classification in insects, 13
Gentians, climatic experiments, 197
Geometrical structure and differentiation, 54,56
Geometrical distinction between germ-cells
and somatic cells, 58
Gladiolus, right and left, 57
Gnophus obscurata, protective colouring, 141
Goldfinch, geographical races, 147
Gonioctena variabilis, variation in sexes of, 121
Gouldian Finch, polymorphism, 148,149
Gracilaria stigmatella, experiments on, 193
Grantia, large varieties of, 125
Ground-Squirrels, local forms of, 132
Grouse, red, variation, 29
Guillemot, Ringed, 150
Guinea-pig, Brown-Séquard's experiments on, 198
Gynandromorphs, 45
Heliconius erato, forms of, 122,164
Helix lapicida, local variation of, 126
striata, 127
Heripensis, 127
Caespitum, 127
trochoides, 127
nemoralis and hortensis, 128
Helminthophila, geographical races of, 157
Hemerophila abruptaria, melanic, 142
Hepialus humuli, in Shetland, 119
Heterostyle plants, 236
Hieracium, 9
Himantopus, 234
Homoeosis, 68
Hybernia progemmaria, 139
Hybrids, sterility of, 233 et seq.
Incompatibility between certain allied species, 239
Individual, geometrical independence of, 58
Inhibiting Factors, 95
Intermediates, nature of, 131,135
Isolation, consequences of, 118
Lacerta muralis, Kammerer's experiments on, 209
fiumana, 210
Leptinotarsa, Power's experiments on, 218
Limbs, extra, in pairs, 72
Limnaea, sinistral, 134
Linaria vulgaris, self-sterility, 239
Loasa fruits, right and left, 57
Lobster, extra claws, 76
Locality, variation connected with, 14,118,146 et seq.,208
Lumbricus, regeneration, 77
Lychnis dioica and vespertina, inter-relations of, 18
macrocarpa, possibly a common parent of, 19
Machetes pugnax, polymorphism of male, 28
Maize, Blaringhem's experiments on, 229
Maize, cumulative factors in, 116
Malformations, dominants, arising de novo, 89
Manx Cat, heredity, 46
Matthiola, 84,104,113
Melanic varieties, 135 et seq.
Memory, analogy with heredity, 190
Meristic variation, 69,83,86
Mirabilis, striped, 57
Models of segmentation, 59,60
"Modes," Coutagne's conception of, 126
Mödling, peculiar race of _Pieris napi_ at, 178
Mole, albino, 27,28
Mule, Linnaeus on, 8
Mutation, Matthioli on, 4
in Mercurialis, 5
in Kales, 5
alleged in bulbs, 5
Theory, 97
periods of, 114
in Bacteria, 214
Mutilation, consequences of, 71
alleged effect of, on offspring, 229
Myxococcus, variation in, 213
Narwhal, asymmetry of tusks, 44
Nemesia strumosa, 91
Neuration, a basis for classification, 13
Nicotiana, sterility of hybrid, 242
Nightjars, varying, 150
Noctuidae, fixity and variability, 25
Noctua, polymorphic and fixed species, 25
Noctua castanea, local forms of, 122
Nomenclature, future of, 94,245
Notonecta, variations of, 130
Odontoptera bidentata, melanic form, 137
Oedipodidae, protectively coloured, 140
Oenothera, new dominant in, 92
rubricalyx and rubrinervis, 92,95
Lamarckiana, 92,101
origin of, 102,244
has bad pollen-grains, 102
factorial analysis of, 103
pollen and egg-cells genetically dissimilar, 104
Oenothera, "twin hybrids", 105
laeta and velutina, 105
reciprocal crosses in, 105 et seq.
possible coupling in, 111
dwarfs, 112,114
"Triple hybrids", 114
alleged variation due to treatment, 227
Ophrys, local variability, 125
Orange, polyembryony, 45
Osmotic growth, 65
Overlapping forms, 146,174
Papilio, geographical races of, 162
Papilio turnus, variation of, 144
Pararge egeria, geographical forms, 166 et seq.
Parthenogenesis, 50
Partula, local forms of, 133
Passer domesticus and montanus, distinctions, 22
Pea, round and wrinkled, 95
Pear, will not cross with apple, 239
Pelargonium, variegated, 55
bud-sports, 56
Periodic phenomena in structure, 63
Peronea, fixed and variable species, 26
"Petites espèces", 248
Petunia, double, 104
Phalanger maculatus, local variation, 119
Pheasant, fixity of, 29
Phigalia pilosaria, melanic, 139,140
Phratora vitellinae, experiments on, 193
Phyllotaxis, 69
Pied varieties common in Passer domesticus
unknown in Montanus, 23
Pieris napi and bryoniae, 174 et seq.
Pig, mule-footed, 46
Pigeon, web-footed, 46,49
Indian Rock, a recessive form, 98
Pigments, nature of, 83
Pisum humile, hybrids with culinary peas, 244
species, 246
Planarian, regeneration of, 71,77
Plotheia frontalis, polymorphic, 26,29
Plusia, fixity and variation in, 26
Poephila gouldiae, variation of, 148,149
Polarity of individual, 44
Polia chi, melanic, 138
Polyanthus, short-styled selected, 236
Polydactylism in Cat, 52,53
Polyembryony, 45
Potato, variation in, 91
Poultry, evolution of, 90
Primula obconica, 91
Primula sinensis, flaked, 57
Leaf-shapes, 70
new dominant in, 92
sterility in, 236
"Giants", 236
Primula, species-hybrids, 242
Protective coloration, 140
Pyrrhulagra, local forms, 120
Python, twin-vertebrae, 60
Quiscalus, geographical races of, 156
Rabbit, Angora, 46
colours of, 93
Incompatibility with hare, 242
Raimannia odorata, Macdougal's experiments on, 226
Rats, Variation in, 248
Recessives, origin of, 90
Reciprocal crosses, giving distinct results, 105 et seq.
Regeneration, 70
Repulsion, 110
Reversal on Regeneration, 77
Rhamphocoelus, geographical forms, 159,184
Rhinoptera, variation in jaws of, 38
Rhythm in repetition, 69
Ribs, variation of, 68
Rights and Lefts, 57-58
Ripples, analogous to segments, 60,66,67
regeneration of, 79
Rollers, geographical races of, 160
Ruff, polymorphism of male, 28
Salamandra, maculosa and atra, 182,199,203
spotted and striped, 207
geographical variation of, 208
Segmentation, nature of, 63
simulated mechanically, 64
compared with rippling, 65
analogies with, 68
Segmentation of normally unsegmented structures, 38
Selection, Natural, an insufficient
cause of definiteness of types, 17,134,142
Sempervivum, 250
Serial Homology, the true nature of, 62,66
Setina, Alpine varieties, 181
Sex of Twins, 44
Sex-factors, possible coupling of, 111
Sexual characters, variation in, 119 et seq.
Siamese twins, 44
Silky Fowl, 84,85
Simocephalus, changed by environment, 218
Sinistral forms, 33-34
Situs transversus, 43
Skate's jaws, variation in, 38
Sloths, vertebral variation, 68
Species, conceptions of, 3,94,99,240,245
allied, distribution of, 185
alternative uses of the term, 245
Specific difference, universality of, 12
of organisms compared with those
of inorganic materials, 15
failure of theory of Selection
to explain, 18,134,247
Sphyropicus varius, 149,156
Spilosoma lubricipeda, varieties of, 181
Zatima, Heligoland form, 181
Spinal nerves, segmentation of, 67
Sporadic variation, 131,134,248
Squashes, polymorphism of, 100
Staphylococcus pyogenes, variation in, 213
Sterility of hybrids, in general, 233
in Lychnis hybrids, 20 et seq.
in crossing forms of Draba, 243
Significance of, 244
Self, 238
Stilt, 234
Stocks, 84,104,113
Striped varieties, 57
Substantive variation, 84
Subtraction-stages, 93
Supernumerary limbs, 72-76
Sweet pea, variation of, 91
sterile anthers in, 237
Symmetry compared with heredity, 41
Symmetry of body approximate, 78
Syndactyly, 47
in foot, 48
Synthetic formulae, in nomenclature, 94
Taeniocampa, fixed and variable species, 25
Tamias, local forms of, 132
Tanagers, geographical races of, 159
Teeth, variation in, 67,39
Tephrosia consortaria and consonaria, 137,139,140
Tephrosia species, separated by season, 119
Terminal members, variation of, 68
Thais rumina, local variation in, 27
Tolerance, persistence of diversity due to, 17,134
Tomato, number of cells in fruit, 46
Transitional populations, rarity of, 165
an example, 178
Tropaeolum, sterile anthers in, 237
Trypanosomes, variation in, 215
Tusk, of Elephant, segmented, 38
of Narwhal, 44
Twinning, 41,44,71
heredity of, 45
in organs, 46
Uria troile, variety of, 150
Vanessa urticae, effects of temperature, 191
Variation, a medley of phenomena, 14,15
sporadic, 131,134
and locality, 118
Causes of genetic, 86,87,131,212
Substantive and meristic, 83
Veronica, specific difference in, 16
intermediates between species, 17
Vertebrae, division in, 60,61
homologies of, 66
Vespa, specific difference in, 23
Vortex, living organism compared with, 40
Wave-motion compared with repetition of parts, 62,67,79
Wheat, cumulative factors in, 116
climatic experiments on, 195
Woodpecker, 234
Zebra, pattern of stripes compared with ripples, 38
INDEX OF PERSONS
PAGE
Ackermann, 242
Agar, 218
Allen, J. A., 132,147,159
Annandale, 47
Arrigoni degli Oddi, 167
Backhouse, 50
Baker, G. T., 166
Bangs, Outram, 120,142,155
Barrett, 26,136,167,173,178,193
Baur, E., 55,99
Baur, G., 119
Beneden, van, 75
Bentham, on species of Veronica, 16
Lychnis, 21
Primula, 22
Bernadin, 42
Bishop, L. B., 153,157
Blaringhem, 229
Bobart, 5
Boisduval, 182
Boissier, 19
Borradaile, 74,75
Boulenger, E. G., 208
Boulenger, G. A., 182,207,209
Boyle, 5,54
Brewster, W., 149,150
Britton, 227
Brown, T. Graham, 198
Brown-Séquard, 197 et seq.
Bruant, P., 51
Buffon, 234
Butler, S., 189,190
Buysson, R. du, 24
Candolle, de, 245
Carpenter, J. H., 172
Chapman, F. M., 148,156,157,158
Chapman, T. A., 13,167,182,231
Church, A. H., 69
Cieslar, 197
Clark, Austin, 142,144
Cockayne, E. A., 43
Cockerell, T. D. A., 224
Compton, R. H., 50,58,227
Cope, 230
Cory, 142
Correns, 239
Coutagne, 125 et seq.
Darwin, on Variation, 1,2
Systematics, 10
Selection, 134,139
Heterostyle plants, 236,237
Darwin, F., 190
Darwin, Sir G., 41
Davenport, 46
Davis, H. M., 102
Delcourt, 130
Deschange, 181
Dobell, 215
Doncaster, 105,121,136
Driesch, 80,81
Duchartre, 51
East, 91,116
Edwards, W. H., 162
Ehrlich, 215
Fellmer, 215
Field, W. L. W., 161
Fischer, E., 192
Fleck, 171,174
Fletcher, W. H. B., 26,181
Foster, Sir N., 39
Gallé, 123
Garrod, 83
Gates, 92,95,102
Gayner, F., 177
Godron, 249
Gold, E., 196
Goldschmidt, 116
Goodwin, E., 137
Gortner, 226
Greene, E. L., 8
Gregory, R. P., 92,100,236
Grenier, 249
Grover, 173
Gruber, 48
Gulick, 119,133
Hamling, 142
Hampson, Sir G., 26
Harris, 142
Hartlaub, 182
Herbst, 42
Heribert-Nilsson, 116
Hewett, 182
Honing, 105
Hunter, John, 44
Jakowatz, 197
Janet, 24
Jeans, 41
Jenkinson, 40
Jentink, 120
Johannsen, 195
Jordan, 185,242,249
Kammere, 199 et seq.
Keeble, 236
Klebs, 250
Krancher, 182
Küchenmeister, 44
Kudicke, 215
Lamarck, 9
Lang, A., 128
Lawrence, W. N., 142,145
Leake, H. Martin, 98,100
Leavitt, 185
Lecoq, 99
Lederer, 167
Leduc, 64,65,80
Leydig, 182
Linden, M. von, 192
Linnaeus, 6,7,8
Lloyd, R. E., 248
Locard, 130
Lock, R. H., 242,244
Loeb, 42,45,50,71,77
Lotsy, 99
Lowe, P. R., 143
Macdougal, W. T., 102,226
Marchant, 7
Mathew, 171
Matthioli, 4
Mayer, A. G., 133
Mendel, Rediscovery of, 2
On Fasciation, 49
Merrifield, 169, 172
Miller, W. D., 120,149
Morgan, 71,77,91,198
Moggridge, 125
Nathusius, S., 242
Nettleship., 44
Newman, H. H., 42
Newsholme, 48
Nilsson-Ehle, 116,169
Norman, A. M., 125,156
Ober, 142
Oberthür, 168,170,193
Oliver, J., 45
Page, H. E., 167,180
Patterson, J. T., 42
Payne, F., 278
Pellew, 236
Poll, 45
Porritt, 136
Poulton, 141
Powers, J. H., 230
Pringsheim, H., 213
Przibram, 72,78,178,194,197,199
Punnett, 110
Ray, 4,5
Raynor, 105
Ridgway, 10,120
Roedelius, 195
Rolfe, 20
Rosen, F., 242
Rosner, 42
Rowland-Brown, H., 167,180
Sargent, 185
Saunders, E. R., 84,104,112
Schima, 177
Schröder, 193,194
Schübeler, 195
Semon, R., 190 et seq.
Sharrock, 5
Shull, 100
Speyer, A., 166,170,181
Spillman, 47
Standfuss, 135,181,191
Staples-Browne, 49,98
Staudinger, 170,179
Stockard, 50,71
Sutton, 236,244
Tornier, 72
Tower, W. L., 218-226
Trechmann, 133
Tugwell, 181
Tutt, J. W. On Definiteness of Species, 13
On Plusia interrogationis, 26
On Tephrosia, 119
On N. castanea, 122
On Pararge egeria, 167 et seq.
Verity, R., 171,177
Vries, H. de, 101-115,222,239
Walker, G, 49
Weir, Jenner, 119
Weismann, 176,188
Wendelstadt, 215
Werbitzki, 215
Werner, 209
Wettstein, 197
Wheeler, G., 168,171
Wheldale, 83
Wilder, 44
Wille, 197
Williams, H., 167,172
Windle, B. C. A., 43
Winslow, 213
Wolf, F., 213
Woodforde, 123
Woltereck, 215
Zeijlstra, 114