VARIATION AND LOCALITY
In all discussions of the modes of Evolution the phenomena of
Geographical Distribution have been admitted to be of paramount
importance. First came the broad question, were the facts of
distribution consistent with the Doctrine of Descent? I suppose all
naturalists are now agreed that they are thus consistent, and that
though some very curious and as yet inexplicable cases remain to be
accounted for, the distribution of animal and plant life on the face
of the earth is much what we might expect as a result of a process of
descent with modification. Passing from this general admission to the
more particular question whether the facts of distribution favour one
special conception of the mode of progress of evolution rather than
another, no agreement has yet been reached. One outstanding feature
is hardly in dispute, namely that prolonged isolation is generally
followed by greater or less change in the population isolated. Groups
of individuals which from various causes are debarred from free
intermixture with other groups almost always exhibit peculiarities,
but on the other hand, cosmopolitan types which range over wide areas
are on the whole uniform, or nearly so throughout their distribution.
Examples of these two categories will be familiar to all naturalists.
The barriers to intercourse may be seas, deserts, prairies,
mountain-chains, or circumstances of a much less obvious character
which isolate quite as effectually. The local unit is not necessarily
an island, a district, or an area of special geological formation,
but may, as every collector knows, be a valley, a pond, a creek, a
"bank" in the sea, a clump of trees, a group of rocks in a bay, or a
particular patch of ground on a mountain side. All the great groups
provide examples of such specially isolated forms. The botanist knows
them well; the conchologist, the entomologist, the ornithologist and the
student of marine life are all equally aware that special varieties or
special species come from special places and from nowhere else. In one
remarkable case the season of appearance plainly acts as the isolating
barrier. _Tephrosia bistortata_ is a small Geometrid moth which has two
broods, appearing in _March_ and _July_ respectively. It is closely
allied to _T. crepuscularia_ which emerges in _May_ and _June_. From the
fact that occasional specimens cannot be quite certainly referred to
one or other of the two, many have held that the two are one species.
Nevertheless, in general they present distinctions which are plain
enough. Some localities have one form only, but in several woods they
co-exist. Experiment has shown that the two can be crossed, and that the
cross-breds can breed _inter se_ and with at least one of the parent
stocks.[1] Some diminution in fertility was observed, but perhaps not
more than is commonly encountered when wild forms are bred in captivity.
In such a case it can scarcely be doubted that the distinctness of
the two forms in the places where they co-exist is maintained by the
seasonal isolation.
Just as the consequences of isolation are to be seen in the most
different forms of life so may they also affect the most diverse
features of organisation, such as size, colour, sculpture, shape, or
number of parts. In the Sloth (_Choloepus_) the geographical races
differ in the number of cervical vertebrae--or in other words, in the
distribution of vertebral differentiation. The geographical races of
_Cistudo_ differ in the number of claws and phalanges.[2]
In Shetland, the males of _Hepialus humuli_ (the Ghost Moth) are
not sharply differentiated in colour from the females, as they are
elsewhere, but in varying degrees resemble them.[3] No such males are
found in other localities, and even in the other Scottish islands they
are normal. In the island of Waigiu the converse phenomenon has been
observed in _Phalanger maculatus_. Generally the male is spotted with
white, and the female is without spots, but in Waigiu the females are
spotted like the males.[4]
The following striking illustration was pointed out to me by Dr. W. D.
Miller. _Euphonia elegantissima_ as it occurs in Mexico and Central
America has the two sexes very distinct from each other. The male has
the lower parts orange and the upper parts a dark indigo blue, with a
bright turquoise-blue head and neck. The female, except for the head,
is of a bright olive green. A form in which the sexes are similarly
differentiated exists in Porto Rico and is known as _E. Sclateri_. But
in many of the other West Indian islands the representative "species"
(_E. flavifrons_) has the two sexes closely resembling the _female_
of _E. elegantissima_. This form is found in Antigua, Barbados, St.
Vincent, and Guadeloupe, from which localities the British Museum has
specimens. All three so-called species are very much alike otherwise.
In the genus _Pyrrhulagra_ (_Loxigilla_) to which Mr. Outram Bangs
called my attention, several distinct and alternative possibilities
occur. The genus has many local species occurring on the various West
Indian islands. These species are characterized by differences in size,
colour, and the shape of the bill. The colours have a narrow range,
being black or greyish, with or without chestnut marks about the head
and throat. In most of the islands the males are in general colour a
full black, and the females are distinctly grey. They are thus found
in San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. In
Porto Rico we meet the peculiarity that the hens are almost as black
as the males (Ridgway describes the black of the hens as slightly less
intense). This form is called _portoricensis_. A larger type, known
as _grandis_, similarly coloured, inhabits St. Kitt's. Then, on the
contrary, in Barbados, _both sexes_ are a dull blackish grey, like the
hens of the Lesser Antilles in general.
The local species of _Agelaius_ show similarly capricious distinctions.
_A. phoeniceus_ is a widely spread species, found over a great part of
North America. The male is black with red-orange bars on the wings, but
the female is somewhat thrush-like in colour. In the island of Porto
Rico there is a form called _xanthomus_, in which _both sexes_ are like
the males of the mainland. A similar species called _humeralis_, also
with both sexes male-like, lives in Cuba. The island of Cuba, curiously
enough, has also a distinct species named _assimilis_, in which the
female is a dull black all over, though the male is like the mainland
type.
So also may local races differ in respect of variability. _Argynnis
paphia_, the Silver Washed Fritillary, through a great part of its
distribution has only one female form. In the English New Forest a
second female form, _valesina_, co-exists with the ordinary _paphia_
female. But in the southern valleys of the Alps the _valesina_ female
is much the commoner of the two, and indeed in some localities where
the species is abundant, I have seen no _paphia_ females in many days
collecting.
The beetle _Gonioctena variabilis_ furnishes an illustration of a
comparable phenomenon affecting the male sex. In 1894 and 1895 I
studied the curious colour variations of this species especially in the
neighbourhood of Granada, and Mr. Doncaster ten years later repeated the
observations on the same ground, and also collected the insect in other
places in the south of Spain. The distinctions are not easy to give in
words and the reader is referred to the colour plate accompanying my
paper.[5] The essential fact is that the males commonly have the elytra
_red with black spots_ and the females for the most part have greenish
grey elytra with black stripes. In some localities a large minority of
males closely resemble the female type, being identical in colour and
then only distinguishable by structural differences. In two Granada
localities I found the proportion of such males quite different. In the
Darro valley about 38 per cent. (in 718) were of this feminine type,
but on the hills some 300 feet above only 19 per cent. (in 3,230) were
like the females. At Castillejo, not far from Toledo I found no such
male in 75 specimens.
Mr. Doncaster collected from several localities, especially from two
areas near Malaga, about 5 miles apart. In one of these the female-like
males were, as usual, in a minority, but in the other these were
actually in great excess, amounting to about 81 per cent. in the 173
taken. Doncaster found a doubtful indication that the composition of the
population varies with the season, which is quite possible, but it is
most interesting to note that in my chief locality after the lapse of
ten years he found the proportions very much the same as I had done at
the same season, for where I had 19 per cent. of the female-like males
his collecting gave 16 per cent. In other respects also, his statistics
corresponded very closely with mine.[6]
The various forms of _Heliconius erato_ are well known to entomologists.
They are strikingly distinguished by the colours of the strong comb-like
marking on the hind wing, which may be red, yellow, green or blue. In
various parts of the distribution in South America sometimes two and
sometimes three of these distinct types co-exist.[7]
The distribution of the varieties of _Noctua castanea_ typifies a large
range of cases. The form which is reckoned the normal of the species
has red fore-wings. It is practically restricted to Great Britain and
Germany, according to Tutt. The other common form, _neglecta_, has grey
fore-wings, and in this pattern it ranges through West Central Europe
from North Italy to Germany. In the British Isles it extends up to
Orkney. In Britain this grey form is by far the commoner, occurring
wherever the species is found. The red form is much scarcer in England,
and does not occur at all in many localities where the grey form is
common. Mr. Woodforde, from whom this account is taken,[8] states that
in August, 1899, he saw considerably over a hundred of the grey in the
New Forest at sugar, but only two red ones. In Staffordshire however the
red is proportionately more numerous and he estimates them as 40 per
cent. of the population. Lastly a form has been taken in Staffordshire
as a rarity in which the red is replaced by yellow, and this has
hitherto been seen nowhere else. It is beyond our immediate purposes
to discuss the genetic relationships of such forms, but the details
of this case are interesting as making fairly clear the fact that the
distinctions between _castanea_ and _neglecta_ are due to combinations
of the presence of and absence of two pairs of factors, of which one
produces a red pigment in the ground colour of the forewing and the
other irrorates the same region with black scales. Mr. Woodforde states
that all intermediates exist, and that in Staffordshire the greys always
have a pinkish tinge. The yellow is doubtless another recessive to the
red.
Species which are uniform in some localities may be polymorphic in
others. Such a phenomenon is well exemplified by the orchid _Aceras
hircina_. Of this species distinct varieties had previously been known
in Germany, but Gallé[9] has lately given a detailed account of a
number of most diverse forms found growing in a district of Eastern
France. Without reference to his plates it is impossible to give any
adequate conception of the profusion of types which the flowers of
the species there assume. In some the lip is elongated to many times
its usual length, twisting and dividing in a fashion suggesting some
of the strangest of the Tropical Orchids. In others the labellum
and the lateral petals are all comparatively short and wide (Fig.
13). Intermediates, combining these qualities in various degrees,
were abundant, and the condition of the species, which was the only
representative of the genus in the locality, recalls the extreme
polymorphism of many of the Noctuid Moths.
[Illustration: FIG. 13. Various forms of _Aceras hircina_. (After
Gallé.) This figure only shows a few of the more striking forms
illustrated in Gallé's plates.]
Somewhat comparable variability has been seen in another Orchid genus
_Ophrys_. In Great Britain the species _apifera_, _aranifera_ and
_muscifera_ though variable are fairly distinct, but Moggridge has
published two series of plates[10] showing a very different state of
things as regards the _Ophrys_ population of the Riviera. Here the
outward diversity is such that the ordinary specific names cannot be
applied with any confidence and the limits of the species are quite
uncertain. It may well be supposed that these Riviera plants are
interbreeding, and indeed we may safely assume that they are. It is,
however, to be remembered that Darwin showed _apifera_ in this country
to be habitually self-fertilised, so that the different behaviour on
the Riviera may itself constitute a local peculiarity. Moreover it is
to be gathered from Moggridge's account that in the districts which he
examined the condition was not to be described by the statement that
our three types were there co-existing and hybridising, but rather we
should say that the population was polymorphic, containing these three
types amongst others. Conchologists are aware that on the Dogger Bank
_Modiola_ attains a size unparalleled elsewhere. The same is true of
the sponges _Grantia compressa_ and _Grantia ciliata_ in the estuary
of the Orwell.[11] Conversely, as we know so well in the case of Man,
dwarf races occur in several special localities. Such examples may be
multiplied indefinitely.
The relation of local forms to species has often been discussed from
many points of view, but I know no treatment of the subject clearer or
more comprehensive than an excellent account of some of the various
manifestations of local differentiation as they appear in Helicidæ
published by Coutagne[12] and a reader interested in the problem which
they raise would do well to make himself acquainted with the original
from which the following notes are taken. He speaks for example of
_Helix lapicida_. This is on the whole a constant form ranging up to the
altitude of 1,300 m., common all over France except at great heights
and in the Olive regions where it is restricted to moist places. Though
subjected to such diverse conditions it shows only trivial variations in
colour and other respects throughout its distribution, excepting that
on both sides of the Pyrenees it has a very distinct sporadic variety
called _Andorrica_ or _microporus_. This variety occurs here and there,
together with the type-form sometimes in colonies (pp. 26-30 and 86).
_Bulimus detritus_ though more restricted in geographical range is a
much more variable form. It exhibits great variations in colour, form,
and size, and as Coutagne well insists, these are independent of each
other. Foreshadowing the methods of factorial analysis he suggests that
distinctions in each respect, the "modes" as he calls them, should
be denoted by a letter, or if desired, by a name, and the several
combinations of differences might thus be most logically and usefully
expressed. Of such combinations he says there are at least 18, all of
which can be found. The whole possible series does not necessarily
occur in the same place, and various localities are characterised by
the presence or absence of certain of the combinations as Coutagne
calls them, and by the relative frequency with which they occur. The
ideas thus enunciated are much in advance of the ordinary practice of
systematists, who give names to forms which are nothing but accidental
combinations of factors, just as the horticulturists for practical
reasons give names to similar combinations, which as we now know are
merely specially noticeable terms in a long series of possibilities.
In each case it is rather the _factors_ which should be named than the
forms which are constituted by their casual collocation. In this special
example of _Bulimus detritus_ the 18 forms are made by the combinations
of three pairs of independent factors. Besides these combinations which
may occur anywhere or almost anywhere in the distribution there are
two more distinct local forms, each of which is regarded by Coutagne
as probably constituting a fresh "mode," perhaps compatible with the
others.
_Helix striata_ (Draparnauld)[13] is truly polymorphic; and its various
forms have been described under various specific names. It abounds in
the calcareous hills of Provence and Languedoc, disappearing in the
alluvial lowlands and equally in the upper levels at about 800-1,000 m.
From this district it extends through regions of similar altitude over a
great part of France (details given).
Locard in his monograph of this group, which he calls collectively the
group of _Helix Heripensis_, tabulates 27 distinct named forms. The
characteristics in which these forms differ have been reckoned as 17,
and as several of these vary in degree of development, the number of
modes may be increased to 109. For practical purposes however Coutagne
considers that the various developments of 7 characteristics in their
several combinations are enough to express the various forms, and he
gives examples of this method of definition. As he observes, though
names may be required to define the modes, no one need be alarmed at
that, for the same names of modes will be applicable to a great range of
distinct species, and the formulae expressing their combinations will
replace the varietal names.
This particular example of polymorphism is but little limited by
locality. Occasional colonies present some special physiognomy which may
in a given place seem almost invariable, though in this very respect the
colonies found elsewhere may be highly variable, but such limitations
are exceptional for _H. striata_.
Some distinct and obvious susceptibilities to the influence of soil
and climate are however noticeable. For example on siliceous ground
the shells are thinner, while on calcareous soils they are thicker;
similarly those from the Northern districts attain a larger size
than those from further South. Moreover those subjected to curtailed
development, whether from drought, heat or cold often show a shortening
of the spire. In contrast with this case Coutagne describes the
varieties of _Helix caespitum_, which he says are for the most part
localised, quoting many illustrative cases.
Another remarkable case in which locality plays a curious part is
provided by the two species _Helix trochoides_ and _pyramidata_. In
France generally they are distinct enough from each other, _trochoides_
being smaller and having a characteristic keel. Coutagne says that after
having collected these species from more than a score of localities he
came upon a colony of _trochoides_ on the island of Pomègues in which
the shells were relatively enormous, most of them having only a slight
keel, and a few none at all. On the other hand he received a consignment
of _pyramidata_ from four localities in Sicily, all small, and one of
them exactly like the _trochoides_ from Pomègues. Judging by the samples
received from Sicily, _trochoides_ is there not more variable than it is
in Provence, while the Sicilian _pyramidata_ is protean.
The relations of the two species _Helix nemoralis_ and _hortensis_
provide an illustration of another kind of manifestation of local
peculiarity. _H. hortensis_ and _nemoralis_ as usually met with, are
two very distinct forms. _H. hortensis_ is smaller and duller, and its
peristome is white. _H. nemoralis_ is larger and more shiny, and its
peristome is brown. In several anatomical points, moreover, especially
in the shape of the dart, there are great differences. For a full
account of these peculiarities of the two forms and a discussion of
their inter-relations the reader is referred to the elaborate work of
A. Lang[14] who has studied them extensively and has also succeeded in
experimentally raising hybrids between them. These hybrids were in a
slight degree fertile with both the parent species, but up to the time
of publication no young had been reared from hybrids _inter se_.
Coutagne describes the result of collections made in 62 French
localities. Some had exclusively _hortensis_, some exclusively
_nemoralis_, and in some the two were found in association. He gives
details of five of these collections from which I take the following
summary of the more essential facts, omitting much that is almost
equally significant.
_Locality A_, near Honfleur. Both forms present, each sharply and
normally distinguished, without any intermediates. They are thus found
in many places. Coutagne instances Müller's observations in Denmark, his
own series from the Jura, etc.
_Locality B._ Vonges (Côte d'Or), 242 _hortensis_ taken at random,
showed 128 with light peristomes (either more or less pinkish or
quite white) and 114 with dark _brown_ peristomes; together with 26
_nemoralis_ all with the usual brown peristomes.
Of the _hortensis_ 50 were in ground-colour _opalescens_ and 1 _roseus_;
and in shape 5 were _umbilicatus_.
_Locality C_, about 3 kilometres from _B_. There were found 35
_hortensis_, of which 20 had light peristomes and 15 brown; together
with 7 _nemoralis_.
Of the _hortensis_ none were _opalescens_; 18 were _roseus_ and none has
the shape of _umbilicatus_.
_Locality D_, about 1,200 metres from _B_. 147 _hortensis_, of which 4
had light peristomes and 143 had brown. No _nemoralis_ were found.
None of the _hortensis_ were _opalescens_ or _roseus_, but 30 were
_umbilicatus_.
In these localities intermediates of every grade existed between the
well-characterised _opalescens_, _roseus_, or _umbilicatus_, and the
other forms, but there were no intergrades between the other _nemoralis_
and the smaller _hortensis_, about which there was no hesitation. In the
next locality a very different state of things was found.
_Locality E._ Banks of the Yvette at Orsay (Seine-et-Oise). The
actual numbers are not given, but we are told that 58 per cent. were
_hortensis_, 33 per cent. _nemoralis_, and 9 per cent. intermediate. As
at Honfleur, the _hortensis_ had white peristomes, and the _nemoralis_
brown. Coutagne's visits to this locality were in 1878 and 1880, and he
calls attention to the fact that Pascal found similar intermediates in
the same neighbourhood in 1873.
The two species, in Coutagne's view, when they occur together, can
generally be sorted from each other with perfect confidence, and it is
only in exceptional localities that these intermediates occur. Whether
they are hybrids, or whether sometimes the species in their variations
transgress their usual limitations is regarded both by Coutagne and
by Lang as a question not yet answerable with certainty. Coutagne
moreover lays stress on the fact that although each species may be
easily known from the other _in its own district_, yet when shells from
different districts are brought together it is sometimes impossible to
sort them. He mentions an example of such casual intermixture occurring
under natural conditions on an island in the Rhone, to which it may
well be supposed that floods had brought immigrants from miscellaneous
localities. This population contained a very large number of uncertain
specimens, and as he says, it was much as if he were to mix the shells
from his 62 localities, after which it would certainly be impossible to
separate the two species again.[15]
Further evidence is given in the same treatise as to other examples
of polymorphism, especially in the genus _Anodonta_, of which Locard
made 251 species for France alone. Here again are cases like those
already given, and many forms or "modes" are found restricted to special
localities, while occasionally in the same locality dissimilar forms are
found, collectively forming a colony, without intermediates.
Taken as a whole the evidence shows the following conclusions to be
true. Local races, whether of animals or plants, may be distinguished
by characters which we are compelled to regard as trivial, or again by
features of such magnitude that if they were known to us only as the
characteristics of a uniform species they would certainly be assumed
without hesitation to be essential for its maintenance. Local forms
may be sharply differentiated from the corresponding populations of
other localities or they may be connected with them by numbers of
intermediates. Not rarely also we find a fact which has always seemed to
me of special significance, that the peculiarity of the local population
or colony may show itself in a special liability to variation, and
this variability may show itself in one of many degrees, either in the
constant possession of a definite aberration, in a dimorphism, or in an
extreme polymorphism.
At this stage attention should be called to two points. First, that
when the details of the geographical distribution of any variable
species are studied in that thorough and minute fashion which is
necessary for any true knowledge of the interrelations of the several
forms, the conception of a species invented by the popular expositions
of Evolution under Selection is found to be rarely if ever realised in
nature.
A species in this generalised sense is an aggregate of individuals,
none exactly alike, but varying round a normal type, the characters of
which are fixed in so far as they are adapted to environmental exigency.
In nature, however, the occurrence of the varieties, and even the
occurrence of the variability is sporadic. In one place a population may
be perfectly uniform. In another it may be again uniform but distinct.
In others the two forms may occur together, sometimes with and sometimes
without intergrades. In some localities a sporadic variety may be an
element of the population, persisting through long periods of time.
In other localities there may be several such aberrations occurring
together which are absent elsewhere.
Secondly, I would remind the reader that in the light of genetic
analysis we know that intergrades, when they do occur, cannot be assumed
to represent conditions through which the species must pass or has
passed on its way to the extreme and definite forms.
Often, perhaps generally, they are nothing but heterozygous forms,
and often also they are conditions corresponding with the presence of
factors in their reduction-stages.
A broad survey of the facts shows beyond question that it is impossible
to reconcile the mode of distribution of local forms with any belief
that they are on the whole adaptational. Their peculiarities are
occasionally the result of direct environmental influence, as we shall
hereafter notice in certain cases, but none can attribute such sporadic
and irregular phenomena to causes uniformly acting.
Writers on systematics, especially those of former generations often
conjecture or assert that local distinctions are caused by "differences
of climate, soil, food, etc.," in vague general terms. It is usually
safe to assume that these remarks do not represent conclusions drawn
from actual evidence, for only rarely can they be translated into more
precise language. So thoroughly have the biological sciences become
permeated with the belief that all distinctions are dependent upon
adaptation, that the mere existence of definite distinctions is felt
by many to be sufficient ground to warrant an assumption that these
distinctions are directly or indirectly due to special local conditions.
For example, Dr. J. A. Allen, who has done so much careful and valuable
work in delimiting the local forms of the United States fauna, writes of
the Ground Squirrels (Tamias)[16] as follows:--
"From the extreme susceptibility of this plastic
group to the influences of environment, it is one of the most
instructive and fascinating groups among North American mammals.
No one can doubt its comparatively recent differentiation from
a common stock, and its dispersion from some common centre.
Whether the type originated at some point in North America, or in
the Northern part of Eurasia, it is perhaps idle to speculate,
but that it has increased, multiplied, spread, and become
differentiated to a wonderful degree in North America is beyond
question; as it is found from the Arctic regions to the high
mountain ranges of Central Mexico, and has developed some twenty
to thirty very palpable local phases."
"Some of them easily take rank as species, others as
subspecies. Probably a more striking illustration of evolution by
environment cannot be cited."
He proceeds to point out that the habits of these creatures are such as
lead to isolation. This may well be admitted, and indeed no exception
can possibly be taken to the passage as a whole, save in the one
respect that there is no real proof that the local diversity is due to
"evolution by environment" or an indication of "susceptibility to the
influences of environment."
Dr. Allen does indeed adduce the fact that California "extending through
800 miles of latitude, with numerous sharply contrasted physiographic
regions, has apparently no less than six strongly differentiated forms,
while the region east of the Rocky Mountains from a little below the
northern boundary of the United States northward to the limit of
trees--a slightly diversified region of at least ten times the area of
California--has only one"! But when one comes to ask how the various
forms are adaptational, and how the influences of environment have led
to their production, only conjectures of a preliminary and tentative
character could be expected in reply. Desert forms are no doubt pallid
as in so many instances, and forest forms are more fully coloured, and
we may readily enough accept such facts as indications of a connection
between bodily features and the conditions of life, but further than
that no one can go; so that when we find size, length of ears or of
tail, the number of dorsal stripes, the pattern of the colours, not to
speak of differences in the pigments themselves, all exhibiting large
modifications, we cannot refer these peculiarities to the causation
of environmental difference, save as a simple expression of faith. I
incline far more to agree with Gulick who, after years of study of the
local variations of the Achatinellidae, came to the conclusion that it
was useless to expect that such local differentiation can be referred to
adaptation in any sense.[17] Even the most convinced Selectionist must
hesitate before such facts as those related by A. G. Mayer regarding
the distribution of _Partula otaheitana_, one of these Achatinellidae.
The island of Tahiti has been scored by erosion so that a series of
separated valleys radiate to the coast. From four successive valleys
Mayer collected the species, and found that in the first (Tipaerui)
valley all the shells were dextral (115, containing 73 young); in the
second valley (Fautaua) 54 per cent. of adults and 55.5 per cent. of
the young contained were sinistral; in the third valley (Hamuta) 69
per cent. of adults and 73 per cent. of young contained in them were
sinistral; and lastly, in the fourth valley (Pirae) all the shells
(131, containing 62 young) were sinistral.[18] In connection with
these observations I may mention the fact that in a certain pond in
the North of England[19] the sinistral form of _Limnaea peregra_ has
been known to occur for about fifty years. Visiting it lately I found
the left-handed shells to be about 3 per cent. of the population. The
species is the commonest British freshwater shell, but left-handed
specimens are exceedingly rare. Will anyone ask us to suppose that the
persistence of a percentage of this rarity in the same place is an
indication of some specially favouring circumstance in the waters of
that pond? It is a horse-pond to all appearances exactly like any other
horse-pond; and I believe that in perfect confidence we may accept the
suggestion of common sense, which teaches us that there is nothing
particular in the circumstances which either calls such varieties into
existence or contributes in any direct way to their survival. Had the
phenomenon of local variation been studied in detail before Darwin
wrote, the attempt to make selection responsible for fixity wherever
found, could never have been made. The proposition that not only the
definiteness of local forms but their variability also is sporadic,
can be established by countless illustrations taken from any group of
either the animal or the vegetable kingdoms. Only exceptionally can the
fixed differences be even suspected of contributing to adaptation, and
sporadic variability, which is a no less positive fact, must manifestly
lie outside the range of such suspicions. It is open to any one to
suggest speculatively that the persistence of special varieties or of
special variability in special places is an indication that in those
places the conditions of life are such that the forms in question are
tolerated though elsewhere the same types are exterminated; but that
consideration, even if it could be proved to be well founded, is not
one which lends much force to the thesis that definiteness of type is
a consequence of Natural Selection. On the contrary, recourse to such
reasoning implies the inevitable but very damaging admission that the
stringency of Selection is frequently so far relaxed that two or more
equally definite forms of the same species can persist side by side.
There is no doubt that this is the simple truth, but when once that
truth is perceived it is useless to invoke the control of Selection as
the factor to which definiteness of type in general must be referred.
The genetic relations of local forms to each other cannot in the
absence of actual breeding experiments be often ascertained. Standfuss
formerly enunciated as a general principle that when two forms
co-exist in the same locality and are able to interbreed, they do not
produce intermediates; but that when the forms are geographically
separated as local races, crosses between them result in a series of
intermediates.[20] In this aphorism there is a good deal of truth, but
if in the light of Mendelian principles we examine the two statements
we see now that the first is in reality only another way of saying
that the distinctness of an aberrational form co-existing with another
is due to segregation, accompanied by some degree of dominance of one
type. Whether, however, one geographically isolated race will give
intermediates when bred with another must depend entirely on the genetic
physiology of the special case, and no general rule can be laid down.
It may well be that, inasmuch as the distinctness of the variety is
maintained by isolation, the difference in factorial composition between
it and the representative form in another area is neither simple nor
sharp; but when two varieties co-exist, though interbreeding, it is now
clear that their differences must depend on the segregation of simple
factors. Plainly such aberrations may in one place co-exist with another
type, and elsewhere be separated from it as local races.
Excellent illustrations of these two stages in evolution are provided
by the melanic varieties of British Lepidoptera. The fact that black or
blackish varieties of many species especially of Geometridae have come
into existence in recent years is well known to British collectors,
and it is not in dispute that they have in several instances replaced
the older type more or less completely in certain districts. In the
year 1900 the Evolution Committee of the Royal Society instituted a
collective inquiry as to the contemporary distribution of these dark
varieties. As the change had happened within living memory and had
greatly progressed in recent years it was hoped that a record of the
existing distribution would serve as a point of departure for future
comparison. The records thus obtained were tabulated by Mr. L.
Doncaster.[21] From that account and from the statements in Barrett's
British Lepidoptera[22] this description of some of the more notable
cases is taken.
The most striking and familiar case is that of _Amphidasys betularia_,
of which only the ordinary type was known in any locality until about
1848-1850, when the totally black var. _doubledayaria_ first appeared
in the neighbourhood of Manchester. This black form was subsequently
recorded in Huddersfield between 1860 and 1870; Kendal about 1870;
Cannock Chase, 1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
1892; Suffolk, 1894; London, 1897. For the Southern Counties of England,
except in the London district, there are still very few records. It
cannot of course be asserted positively that the variety spread from its
place of first appearance into the other localities, and that it did not
arise _de novo_ in them, but there can be little doubt that the process
was one of colonisation. On the European Continent the first records are
from Hanover in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
Dresden about the same date.
As regards the increase of the variety we have the fact that in
Lancashire, Cheshire and the West Riding of Yorkshire the black is now
the prevalent form; and in some places, as for example, Huddersfield,
the black alone is now found, though it was unknown there till between
1860 and 1870. About 1870 at Newport, Monmouth, the two forms were in
about equal numbers, but a few years later the type had almost vanished.
Similarly in Crefeld, where the black form was still very rare in the
eighties, it now forms about 50 per cent. of the population. In the
London district the black remains scarce and at the date of the report
it was still very scarce. From Ireland there is only one record and
there are hardly any from Scotland.
_Boarmia repandata_ is another species which is behaving in a somewhat
similar way. Unlike _betularia_, however, the species is a variable
one, and has several colour-forms, amongst them the banded var.
_conversaria_, and many others. In addition to these there is a
black form in the North of England which seems to be spreading. In
Huddersfield the black was first recorded in 1888, and in 1900 20-25 per
cent. were black. At Rotherham the black or very dark are now prevalent
and have increased in the last 15 years. From the Midlands, East Anglia
and Southern Counties the returns show only the light and medium forms.
Of _Odontoptera bidentata_ several intergrading dark forms exist, and
these are found exclusively in the North and the Midlands. Unicolorous
blacks have been found recently in the Lancashire mosses and at
Wakefield. At Huddersfield 50 years ago the light forms were prevalent,
but now a rather dark brown, not infrequently suffused with black, is
the commonest. In Southern Counties only light forms are known.
_Phigalia pilosaria_ in South England is always light, but in the North
the prevalent form is darker. About 35 years ago a form with unicolorous
sooty fore-wings and dull grey hind wings was first seen in Yorkshire
and a similar form is now taken regularly in South Wales.
In the following cases the dark varieties were found originally only in
the South.
_Boarmia rhomboidaria_ gave rise about 40 years ago to a unicolorous
smoky variety called _perfumaria_. This was at first peculiar to the
London district, but it has since been taken in Birmingham and other
large cities. More lately coal-black specimens have been found at
Norwich, and others similar but hardly so dark were taken in the South
of Scotland and at Cannock Chase.
_Eupithecia rectangulata_ is a similar case. Formerly the light forms
were prevalent but within sixty years they have almost entirely been
replaced in the South of London by a nearly black form.
_Tephrosia_ (_Boarmia_) _consortaria_ and _Tephrosia consonaria_ are
exceptionally interesting, for they have both given off dark forms in
the same wood near Maidstone, which is far from the usual "centres of
melanism." They were discovered in this locality by Mr. E. Goodwin. That
of _consortaria_ is a dark grey, but that of _consonaria_ is a full
black, and nothing like either has been found anywhere else.
These examples are all taken from the Geometridae but others, though
of a less conspicuous kind, could be given from the Noctuidae or the
Micro-Lepidoptera. _Acronycta psi_, for instance, has a suffused form
which is believed to be becoming more frequent in the London district.
_Polia chi_ has two dark forms, _olivacea_, a yellowish grey with dark
markings, and _suffusa_ which is a darker, blackish-slate colour. Both
occur in the North of England, sometimes together, sometimes separately,
or mixed with the type and many intermediates. The distribution is
peculiarly irregular. At Huddersfield, where the very dark form appeared
suddenly about 1890, some 30 per cent. are said to be now dark and about
6-7 per cent. very dark, but at Saddleworth, 12 miles away, only the
pale forms occur.
Several questions of interest arise in regard to this evidence. This
progressive Melanism has arisen in certain families only, and may be
confined to certain species only, within those families. As in almost
all other examples in which variation has been much observed, its
incidence is capricious and specific. A collateral line of inquiry
relates to the degree of discontinuity which the variation manifests.
Here again there is no rule. Generally speaking, in _A. betularia_,
to take the case most fully studied, the variation is discontinuous.
Real intermediates between _betularia_ and _doubledayaria_ are in most
localities absent or rare. The black spots of _betularia_ may often
be larger or more numerous than in the normal, but this variation has
nothing to do with _doubledayaria_, and is not an intermediate stage
towards it, though sometimes wrongly so described. _Doubledayaria_ owes
its characteristic appearance to a factor which blurs the surface of the
wings with a layer of black. Sometimes this blurring is slighter than
in the real _doubledayaria_, and these forms are real intermediates.
Occasionally the fore-wings alone are thus blurred. These intermediates
are clearly due to reduction-stages of the _doubledayaria_ factor,
and are related to it as a blue mouse is to a black, or a dutch
rabbit to a self-colour. It cannot positively be asserted that the
full _doubledayaria_ existed before the intermediate, but it almost
certainly did. In certain places as for instance in Belgium, there is
evidence that intermediates have at various times been fairly abundant,
but they have never become common, nor are they known to exist in the
absence of _doubledayaria_. When the black variety and the light type
breed together they do not usually have intermediates among their
offspring, and the evidence is consistent with the view that the
black is a complete dominant. The same is probably true of _Tephrosia
consonaria_.
In some of the other species we know that the darkest forms did
not appear first. For example in _Phigalia pilosaria_ and _Boarmia
rhomboidaria_ dark forms existed and are believed to have increased in
number before the darkest made its appearance. _Hybernia progemmaria_ is
said to have become darker gradually both in Cheshire and in the West
Riding, and a uniformly smoky variety appeared in South Yorkshire less
than 45 years ago which has spread to neighbouring counties. The dark
medium has become the commonest form in Huddersfield district, where the
very dark variety is now about 20 per cent. of the population, though
the light form is still common.
Taking the evidence together we find it consistent with the view that
dark forms have appeared sporadically, in some species the very dark
appearing first and intermediates later, in others the moderately dark
came first and the darkest later in time. It is practically certain that
the change has in general come about not by a gradual change supervening
on the population at large, but by the sporadic appearance of dark
specimens as a new element in the population, and strains derived from
these dark individuals have gradually superseded the normal type more or
less completely.
If it could be shown that these melanic novelties had a definite
advantage in the struggle for existence they would provide an instance
of evolution proceeding much in the way which Darwin contemplated. The
whole process would differ from that conceived by him as the normal
method of evolution only in so far as the change has come about with
great rapidity and in some instances largely by the appearance and
success of discontinuous varieties. The question, however, must be
asked whether the dark form can reasonably be supposed to have an
advantage by reason of their darkness. Some naturalists believe that
the darkness of the colours does thus definitely contribute to their
protection by making the insects less conspicuous and thus more likely
to escape the search of birds. In support of this view it may be
pointed out that it is in the manufacturing districts of Lancashire and
Yorkshire, and again in the London area that the melanics have attained
their greatest development. Consistently with this argument also, it is
in the neighbourhood of Crefeld and Essen, the black country of Germany,
that they have chiefly established themselves on the Continent, and
_Phigalia pilosaria_ in the black form is now at home in South Wales.
Thus superficially regarded, the evidence looks rather strong, but it is
difficult to apply the reasoning in detail. We have first the difficulty
that the black form of _betularia_ for instance has established itself
in thoroughly rural districts, notably near King's Lynn in Norfolk,
and in the neighbourhood of Kendal and Windermere. The black form
of _consonaria_ and the dark _consortaria_ appeared in a wood near
Maidstone, far from town smoke, and the black _rhomboidaria_ was first
found at Norwich, which, as towns go, is clean. Then again the spread of
the melanics is very irregular and unaccountable. The black _pilosaria_
is found both in the West Riding and in the Swansea district, but
not yet elsewhere. It rapidly increased at Huddersfield, but made no
noticeable progress at Sheffield though recorded there for ten years.
It is also a remarkable fact that no similar melanic development has
been observed in America, and, so far as I am aware, comparable melanic
varieties have not appeared on the European continent except in the case
of the few sorts which possibly may have come from England.
The whole subject is beset with complications. It must not be forgotten
that in a few species of moths there is an obvious and recognised
conformity between the colours of the perfect insect and that of the
soil on which they live, comparable with that which is so striking in
the case of some Oedipodidae and other grasshoppers. Of this phenomenon
the clearest example is _Gnophos obscurata_, which is a most variable
species with many local forms. Of these a well-known dark variety lives
on the peaty heaths of the New Forest and other districts, but on the
chalk hills of Kent, Sussex and Surrey various light varieties are
found, of which one is a bright silvery white, very near in colour to
the colour of a chalky bank. This case does not seem to be one of direct
environmental action,[23] for Poulton found no change induced by rearing
larvae among either white or black surrounding objects. No one however
can doubt that there is some indirect connection between the colour of
the ground and that of the moths.
To my mind there is a serious objection to the theory of protective
resemblance in application to such a case as that of the _betularia_
forms, which arises from the fact that the black _doubledayaria_ is a
fairly conspicuous insect anywhere except perhaps on actually black
materials, which are not common in any locality. Tree trunks and walls
are dirty in smoky districts but they are not often black, and I doubt
whether in the neighbourhood of Rotherham, for instance, which is one
of the great melanic centres, _doubledayaria_ can be harder for a bird
to find than _betularia_ would be. After all, too, many of the species
much affected are not urban insects. They live in country places between
the towns, and the general tone of these places even in Lancashire
and the West Riding is not very different from that of similar places
elsewhere. As against the objection that the black varieties are much
blacker than the case requires it may be replied that we know nothing
of the senses of birds, and that perhaps to their eyes blackness does
constitute a disguise even though the surroundings are much less dark.
This is undeniable, but recourse to such an argument is dangerous; for
if the sight of the insect-eating birds is so dull that it does not
distinguish dark things from dingy grey, we cannot subsequently regard
the keen sight of birds as the sufficient control which has led to the
minute and detailed resemblance of many insects to their surroundings.
Those who see in such cases examples of the omnipotence of Selection
must frequently find themselves in this dilemma.
Taking the evidence as a whole, we may say that it fairly suggests the
existence of some connection between modern urban developments and the
appearance and rise of the melanic varieties. More than that we cannot
yet affirm. It is a subject in which problems open up on every side,
and all of them are profitable subjects for investigation. Unhappily
such animals are difficult to rear successfully in captivity for many
generations, owing to their extreme liability to disease. Not the least
interesting feature of the melanics is the fact that the black varieties
provide about the best and clearest example of a new dominant factor
attaching itself to a wild species in recent times. None of the cases
are satisfactorily recorded or analysed as yet, but the evidence is
clear that _doubledayaria_ is a dominant to its type, and in several
other dark varieties, though the pigment deposited is not black, the
records show that the increased amount of the pigment almost certainly
is due to a positive factor. Of this, _Hemerophila abruptaria_ is a
good example.[24] There are some irregularities in the results, but
taken together they leave little doubt that the dark brown variety is a
dominant and the light, yellowish brown a recessive.
A curious parallel to the rise of the melanic moths in England
is provided by the case of the Honey-creepers or Sugar-birds, in
certain West Indian islands.[25] These birds of the genus _Coereba_
(_Certhiola_) range from Southern Mexico to the Northern parts of
South America and through the whole chain of the West Indian islands
and Bahamas except Cuba. There are numerous local forms, and many of
the islands have types peculiar to themselves, as is usual in such
cases. Some of the types or species range through several islands, but
according to Austin Clark[26] no island has more than one of them.
Cory[27] reckoned twelve such species within the Antillean region. They
are small birds about the size of a nuthatch with a general colouring of
black, yellow, and white. From the island of St. Vincent the Smithsonian
Institution received in the late seventies of last century several
completely black specimens in addition to two of the usual type of
colouring. The black were described by W. N. Lawrence as _atrata_, and
those marked with the usual yellow and white were called _saccharina_.
The collector (Mr. F. A. Ober) reported that the black form was common,
and that the _saccharina_ form was rarer. Lawrence remarks, "Had there
been only a single example (of the black form) I should have considered
it as probably a case of abnormal colouring, but it seems to be a
representative form of the genus in this island."[28] There is of course
no doubt of the correctness of the view taken by Austin Clark that
"_atrata_" is a black variety. The black bird is in every respect, other
than colour, identical with _saccharina_, and it is even possible to
detect a greenish colour in the areas which would normally be yellow,
showing plainly enough the yellow pigment obscured by the black.
We have next the interesting fact that like our melanic moths the dark
form is replacing the "type." At the time of Ober's visit the type was
already in a minority, but now it is nearly or perhaps actually extinct,
though the black form is one of the commonest birds on the island.
Austin Clark found no specimen when he collected there in 1903-4, though
formerly it was not uncommon in the vicinity of Kingston and in the
immediate windward district of St. Vincent.
The Grenadines are geographically just south of St. Vincent, though
separated by a deep channel. In these islands no black forms have
yet been taken, but Grenada, the next island to the south, has both
normals and blacks. There are trifling differences of size between the
Grenada birds and those from St. Vincent, the Grenada specimens being
slightly smaller and for this reason they have received distinct names,
the form marked with yellow and white being called _Godmani_ (Cory)
and the black, _Wellsi_ (Cory), but this merely introduces a useless
complication. There is evidence that in Grenada, as in St. Vincent,
the black is gradually ousting the original type, but the process has
not gone so far as in St. Vincent. Austin Clark very properly compares
this case of the Sugar-birds with that of _Papilio turnus_, which
as is well-known, has a black female in the southern parts of its
distribution, in addition to a female of the yellow type, but in the
Northern States the black female does not occur.
During the present year P. R. Lowe, who lately studied _Coerebas_ on
a large scale in the West Indies, has published an important paper on
the subject.[29] He calls attention to the fact that Cory recently
found a black form of _Coereba_ on Los Roques Islands, and he himself
discovered another on the Testigos Islands. Both localities are on the
coast of Venezuela, far from St. Vincent and Grenada. The whole problem
is thus further complicated by the fact that the black varieties have,
as we are almost driven to admit, arisen independently in remote places.
Improbable as this conclusion may be, it is still more difficult to
regard all the black forms as derived from one source. For first, they
present definite small differences from each other; and secondly we have
to remember a consideration of greater importance, that the very fact
that each island has its own type must be accepted as proving that the
localities are effectively isolated from each other, and that migration
must be a very rare event.
The rarity of such illustrative cases is, I believe, more apparent than
real. It is probably due to the extreme reluctance of systematists to
admit that such things can be, and of course to the almost complete
absence of knowledge as to the genetic behaviour of wild animals and
plants. Only in such examples as this of the _Coereba_, where colour
constitutes the sole difference, or that of the moths which have been
minutely studied by many collectors, does the significance of the facts
appear. The arrangement of catalogues and collections is such that much
practical difficulty of a quite unnecessary kind is introduced. For
example, in this very case of _Coereba_, I find the British Museum has
a fine series from Grenada including 3 normals and 11 black, and also
16 blacks from St. Vincent. If the black specimens from Grenada were
put with the normals which are almost certainly nothing but a recessive
form of the same bird, the variation would strike the eye on even a
superficial glance at the drawer. But following the notions so naively
expressed in the passage quoted above from W. N. Lawrence, the blacks
from Grenada are put apart together with the other blacks from St.
Vincent, though two of them were shot on the same date as one of the
normals.
FOOTNOTES:
[1] For the evidence see Tutt, J. W., _Trans. Ent. Soc._, 1898, p. 17.
Compare the remarkable case given by Gulick (_Evolution Racial and
Habitudinal_, p. 123) of the two races of _Cicada_, which are separated
by reason of their life-cycles, one having a period of 13, the other 17
years.
[2] For references see _Materials_, p. 396, and also G. Baur, _Amer.
Nat._, 1893, July, p. 677.
[3] Jenner Weir, _Entomologist_, 1880, XIII, p. 251.
[4] Jentink, _Notes Leyden Mus._, 1885, VII, p. 111. Specimens
illustrating this peculiarity are in the British Museum.
[5] _Proc. Zool. Soc._, 1895, p. 850. Plate. Many points beyond that
mentioned above are involved in this remarkable case. For example, not
only are there males like females, but a small proportion of females
resemble the ordinary male type. The stripes are not merely the spots
produced, for they occupy different anatomical positions. The spots
almost always go with a black ventral surface, but the striped forms
nearly always have that region testaceous. _Spartium retama_, the
food-plant, will not grow in England, but if it could be naturalised in
America the whole problem might be investigated there and results of
exceptional interest would almost certainly be attained.
[6] Doncaster, L., _Proc. Zool. Soc._, 1905, II, p. 528.
[7] I am not aware that the details of this striking case have ever
been worked out. It should be noted that the green and blue forms are
not due to simple modification of the red pigment; for these colours,
due to interference, fork over the area occupied by the red lines. The
distinctions between these forms cannot therefore be simply chemical,
as we may suppose them to be, for instance, in the case of many red
and yellow forms, and the genetic relationships of the _Heliconid_
varieties would raise many novel problems and be well worth studying
experimentally.
[8] Woodeforde, F. C., _Trans. North Staffordshire Field Club_, XXXV,
1901, Plate.
[9] E. Gallé, _Compte Rendus du Congres Internat. de Bot. a l'Expos.
Univ._, 1900, p. 112.
[10] Flora of Mentone, 1864-8, _Nova Acta Acad. Caes._, XXXV, 1869.
[11] I owe these facts to Canon A. M. Norman, who showed me illustrative
specimens. They were originally described by Bowerbank (_Monogr. Brit.
Spongiadae_, vol. II, pp. 18 and XX; vol. III, Pls. I and III). A
specimen of _G. compressa_ measured 5 inches, with a greatest width of
3-1/4 in. _G. ciliata_ was found measuring 3 in. long and 3/4 in. wide.
These dimensions are many times those of normal specimens.
[12] Coutagne, G., _Recherches sur le Polymorphisme des Mollusques de
France_, _Annales Soc. d'Agric. Sci. et Industr. Lyon_, 1895.
[13] As to the synonymy and references see Coutagne, p. 45.
[14] A. Lang, _Die Bastarde von H. hortensis Muller H. nemoralis L._
Jena, G. Fischer, 1908; with a fine coloured plate showing the varieties
of the species and their hybrids.
[15] With this evidence compare that given by A. Delcourt in his
valuable papers lately published relating to the variations of
_Notonecta_. See especially _Bull. Sci. Fr. Belg._, 1909, XLIII, p. 443;
and _C. R. Soc. Biol._, 1909, LXVI, p. 589.
[16] Allen, J. A., _Bull. Amer. Mus. N. H._, III, 1891, pp. 51-54.
[17] J. T. Gulick, _Evolution, Racial and Habitudinal_, Carnegie
Institution, Publication No. 25, 1905.
[18] A. G. Mayer, _Mem. Mus. Comp. Anat. Harvard_, Vol. XXVI, 1902, p.
117. From the tables given I cannot ascertain the actual numbers from
the two intermediate valleys, but they were considerable.
[19] To which I was very kindly guided by Mr. C. T. Trechmann.
[20] Standfuss, _Handbuch d. paläarkt Gross-schmet_, 1896, p. 321.
[21] _Ent. Rec._, XVIII, No. 7, 1906.
[22] This evidence was largely collected by Mr. G. T. Porritt, who has
given much attention to the subject.
[23] Such direct action has of course been proved to occur in the case
of several dimorphic larvae (_e. g._, _A. betularia_, itself) and pupae.
[24] See Harris, _Proc. Ent. Soc. London_, 1904, p. lxxii, and 1905, p.
lxiii; also Hamling, _Trans. City of London Ent. Soc._, 1905, p. 5.
[25] I am indebted to Mr. Outram Bangs of the Harvard Museum for calling
my attention to this remarkable case.
[26] _Auk_, 1889, VI, p. 219.
[27] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
[28] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
[29] _Ibid_, 1912, pp. 523-8.