LOCALLY DIFFERENTIATED FORMS. _Continued._
CLIMATIC VARIETIES
In this chapter we will examine certain cases which illustrate phenomena
comparable with those just considered, though as I have already
indicated, they form to some extent a special group. The outstanding
fact that emerges prominently from the study of the local forms is that
when two definite types, nearly allied, and capable of interbreeding
with production of fertile offspring, meet together in the region
where their distributions overlap, though intergrades are habitually
found, there is no normally or uniformly intermediate population
occupying the area of intergradation. Such phenomena as these must, I
think, be admitted to have great weight in any attempt to construct a
theory of evolution. True we must hesitate in asserting their positive
significance, but I see no escape from the conclusion that they throw
grave doubt on conventional views. Again and again the same question
presents itself. If _A_ and _B_ lately emerged from a common form why is
that common form so utterly lost that it does not even maintain itself
in the region of overlapping? Almost equally difficult is it, in the
cases which I have numerated, to apply concrete suggestions based on
any factorial scheme. We may see that in _Heliconius erato_ the type
with the red mark on the hind wing probably contains a dominant factor,
and that where the red mark is absent the metallic colours are exposed;
and that similarly the green metallic colour may have another factor
which distinguishes it from the blue. In this way we can fairly easily
represent the various types of _erato_ on a factorial system as the
result of the various possible combinations of two pairs of factors. But
there we stop, and we are quite unable to suggest any reason why one
area should have the red and the green type while another should have
the blue also. So again with _Colaptes_ or the Warblers. By application
of a factorial system, admittedly in a somewhat lax fashion, the
genetic interrelations of the types can be represented; but how it comes
about that each type maintains a high degree of integrity in its own
region we can only imagine. Each has in actual fact a stability which
the intermediate forms have not, but we cannot yet analyse the nature
of that stability. Mendelian conceptions show us how by segregation the
integrity of the factors can be in some degree maintained, but not why
certain combinations of factors should be exceptionally stable. All that
is left us to fall back on is the old unsatisfying suggestions that some
combinations _may_ have greater viability than others, that there _may_
be a tendency for like to mate with like, and so forth.
These difficulties acquire more than ordinary force in those cases in
which the two fixed types inhabit regions differing in some respect
so obvious and definite that we are compelled to regard each type as
climatic and as specially adapted to the conditions. When for example
an animal has a distinct type never met with except in Arctic or Alpine
conditions, and another type proper to the plains and temperate regions,
what are the characteristics of the population of intermediate latitudes
or at intermediate levels? Some of the examples discussed in the last
chapter may be instances of this very nature, but even if they are not,
others are forthcoming which certainly are. The evidence of these cases
leads to the suspicion that with further knowledge they will be found
to consist of two classes, some in which the observer as he passes from
the one climate to the other will find the intermediate area actually
occupied by a population of intermediate character, and others in which,
though we may presume the maintenance of intermediate conditions in
the transitional area, there is no definite transitional population.
This interrupted or discontinuous distribution seems, so far as I have
means of judging, to be by far the more common of the two. I do not
doubt that by sufficient search individuals representing every or almost
every transitional form can be found, but it is apparently rare that
_populations_ corresponding to these several grades can be seen. The
question has in few if any cases been studied with precision sufficient
to provide a positive answer; but I suspect that real and complete
continuity, in the sense thus defined, will only be found where the
character of the local populations depends _directly_ on the conditions
of life, and shows an immediate response to changes in them apart from
that postponed response which we suppose to be achieved by selection.
Obviously the character must be one, like size for instance, capable of
sensibly complete gradation.
The only example I have met with of the phenomenon of anything like a
complete intergradation between local types really distinct in kind is
that provided by the butterfly _Pararge egeria_. It is well known to
entomologists that this insect exists in two very different types, a
northern one, the "Speckled Wood" of England, in which the spots are
a pale whitish yellow, and a southern type having the full fulvous
colour that we know as characteristic of _megaera_, the "Gatekeeper."
It appears that Linnaeus gave the name _egeria_ to the southern
type,[1] and our own is now called _egerides_. Broadly speaking, so
far as Great Britain, France, and the Spanish Peninsula are concerned,
the tawny-coloured _egeria_ occupies Spain and western France up to
the latitude of Poitiers and the pale yellow _egerides_ extends from
Scotland, where it has a scanty distribution, through southern England,
where in suitable localities it is common, and the north of France
to Paris.[2] The two types when placed side by side are strikingly
different from each other, and are an excellent illustration of what is
meant by climatic variation. The insect is not a great traveller and
probably scarcely ever wanders far from its home. It should therefore
be possible by collecting from north to south to find out how the
transition is effected, whether suddenly or gradually. This at various
times I have endeavoured to do, but I am still without exact information
as to the population in certain critical areas. In addition to the
information derived from specimens which I have collected or seen
in the collections of others there is a good account of the general
distribution in Europe given by the Speyers,[3] who evidently paid
more attention to the subject than most lepidopterists have done, and
many more recent records. In particular Oberthür[4] has published many
details as to the distribution in western France and I am especially
indebted to Mr. H. Rowland-Brown for a long series of notes as to the
distribution in France generally, and to Mr. H. E. Page and Dr. T. A.
Chapman, Mr. Oberthür Prof. Arrigoni degli Oddi, Mr. H. Williams and
other correspondents, for showing me forms from many localities. The
butterfly is attached for the most part to woods of deciduous trees and
to country abounding in tall hedges or rough scrub. It is not usually to
be found in highly cultivated districts or in very dry regions. Hence
there is necessarily some want of continuity in the distribution at the
present time and I should think a mile or two of arable land without big
hedges would constitute a barrier hardly ever passed. The larva feeds
on several coarse grasses, especially _Dactylis glomerata_. Barrett
mentions also _Triticum repens_. In this country the winter is usually
passed in the larval stage, but I have found that in captivity, at
least, there is much irregularity. The larvæ feed whenever the weather
is not very cold and may pupate, but if sharp cold comes on when they
are pupating or nearly full-grown they often get killed unless protected.
Some writers speak of a difference between the early and later broods,
but I have never noticed this, and I do not think that the general tone
of the yellow is affected by the seasons (see Tutt, _Ent. Rec._, IX,
1897, p. 37).[5]
Beginning at the south of Spain the thoroughly fulvous type _egeria_
is common at Gibraltar in the Cork woods, at Granada, and doubtless
generally. Lederer is said to have found only this type in Spain
(Speyer), and though I have no precise information as to other places
in the Peninsula north of Jaen I feel tolerably sure that there is no
change from south to north.[6] Immediately north of the Pyrenees we
still meet _egeria_ exclusively, and up to Poitiers at least there is no
noticeable change. But somewhere between Poitiers and the bottom of the
Loire valley at Tours, the genuine southern type comes to an end, and
the whole population begins at the Loire to be of an intermediate type,
easy to distinguish both from _egeria_ and from _egerides_. As to the
exact condition of the species in the fifty miles separating St. Savin
on the Vienne from places on the Loire I have no adequate information. I
have only one small sample from there, but it does contain insects both
of the southern and intermediate types taken on the same day, in a wood
near Preuilly. Oberthür also states that at Nantes the true southern
form exists in company with the northern. From this I infer that the
southern form extends up the coast further than it does inland, but
I imagine the representative spoken of as northern would be of usual
Brittany or intermediate type.
The Vienne river joins the Loire, so the true southern type reaches over
into the basin of the Loire. From the Loire (Tours, Corméry) north to
Calvados (Balleroy) only the intermediate is found, so far as I know,
and the same type extends over Brittany.[7] In general, however, the
woods near Paris have the thoroughly northern type _egerides_, but at
St. Germain-en-Laye and at Etampes (Oberthür) the population approaches
the intermediate type.
On the whole the intermediate type is certainly less homogeneous than
either of the extremes, and females with the two central spots either
paler or more fulvous than the rest are not uncommon, but I have never
taken one on the Loire or in Brittany which I should class with either
of the extreme types.
Before speaking of the distribution in other parts of France and in
Europe generally I will briefly state the results of my breeding
experiments. The work was done many years ago before we had the
Mendelian clue, and it is greatly to be hoped that some one will
find opportunities of repeating it. Crossing the English and the
thoroughly southern type the families produced agree entirely with
the intermediates of Brittany and the Loire. Reciprocals are alike.
Of F_{2} I only succeeded in raising very few and of those that I had
(about 30) nearly all were intermediate in character, though perhaps
rather less uniform than F_{1}. One family alone, containing only 4
specimens, had one _egerides_, and three fulvous intermediates. As the
case stands alone I hesitate whether or not to suppose it due to some
mistake. Moreover from F_{1} crossed back with the respective parental
types I had fairly long series, especially from F_{1} × the southern
type, and looking at these families I cannot see any clear evidence
of segregation. On the contrary, I think that though there are slight
irregularities, they would, taken as a whole, be classed as coming
between the intermediate type and the extreme form used as the second
parent. This at least is true when the second parent was of the southern
type.
On this evidence I have regarded the case as one in which there is
no good evidence of segregation and as conforming most nearly with
the conventional view of gradual transition in response to climatic
influences. Such influence must however be indirect; for I reared five
generations of the northern type in England, and these, though they
included several abnormal-looking specimens in the last generation and
then died out, did not show any noticeable change from the fulvous
colour of the wild type. Merrifield[8] also found that heat applied to
pupae of the northern type produced no approach to the southern type.
Looking at the facts now in the light of more experience it seems to me
just possible that the case may be one in which, as in Nilson-Ehle's
Wheats, the dominant differs from the recessive in having two pairs of
factors with similar effects. The fulvous type for example may have
two or more elements in separate pairs which together produce the full
effect, and the intermediate may have one of these. If this were so,
some segregation should of course eventually be observable, but the
proportion of the various fulvous and fulvous-intermediate individuals
would be large, and the reappearance of actual representatives of the
northern type might be rare. I admit that this is a somewhat strained
interpretation of the facts, and as yet it is not entitled to serious
consideration. Nevertheless I am led to form some such expectation
partly from the great difficulty in the way of any other, partly from
the evidence of the small mixed sample found at Preuilly and partly from
the statements given by Oberthür. There are moreover other features in
the general distribution of the species which make it improbable that
the dependence on climate can after all be so close. Published lists
are unfortunately of little use in deciding which form occurs at a
particular place, because, since the name _Meone_ has ceased to be used
for the southern form, there is no complete unanimity among authors as
to the application of the names _egeria_ and _egerides_, and unless
more particulars are given, either name may be used for either form.
Besides this, difficulty arises from the fact that the intermediate
type is not generally distinguished at all, and English collectors
finding it, may easily record it as the southern type. From Staudinger's
note on the distribution, I gather that he, on the contrary, reckoned
the intermediate with the northern type, as do the Speyers also. The
late Mr. J. W. Tutt was careful to distinguish the three forms and has
left several useful records. Easy therefore as it might seem to be to
make out the distribution of such a familiar insect in its various
modifications, there are serious practical difficulties, and until
long series are brought together with this special object in view many
obscurities will remain.
With only the series from England, the west of France, and Spain before
one it would be easy to regard the successive series of tones as a fair
measure of climate; the brighter the colour, the hotter might one expect
the locality to be. Such rough correspondence is often to be observed
in butterflies and birds. It becomes impossible to take these simple
views in the light of more complete knowledge. Beginning with France
the fulvous _egeria_ occupies the lower valley of the Rhone, probably
from well above Lyon, though I have no exact information respecting the
country above Avignon. According to Speyer it also takes the department
of Lozère. The same authority says that Puy-de-Dôme has "_egeria_,"
meaning perhaps the intermediate form, with the fulvous form much less
commonly. Next comes the curious fact that though the Lower Rhone
(Avignon, Tarascon, Nîmes) has the true fulvous form, Hyères, Cannes,
Grasse, Nice, Digne, and Alassio have _the intermediate_. Savoy has the
intermediate (Chambéry) and even _egerides_ perhaps, though in the same
latitude on the west of France there is nothing but the fulvous type.
At Chalseul and Besançon (Doubs) the ordinary northern type is found.
Switzerland generally, I believe, has the northern type, but Staudinger
gives _egeria_ for Valais and the intermediate occurs in Vaud.[9] The
south side of the Alps has probably colonies of the pale _egerides_, and
of intermediates. Orta, with a very hot summer, has the English type
(Tutt, _Ent. Rec._, XII, 1900, p. 328). Locarno has the intermediate
(_ibid._, XV, 1903, p. 321). North Italy in general and western
Piedmont have the intermediate; but further south _egeria_ begins,
at what region I do not know. Speyer gives on his own authority the
remarkable statement that at Florence both extremes occur, but chiefly
intermediates between the two. Mr. R. Verity however kindly informs
me that in his experience this is not so, and that neither the real
southern type nor the northern occur there. Sardinia, Sicily, Crete all
have the southern type. Greece probably has various types. Staudinger
(_Hor. Ross._, VII, 1870, p. 78) says intermediates resembling Nice
types common everywhere, but from "Greece" the British Museum has a
series that would pass for English specimens; and the same type occurs
near Constantinople. The island of Corfu has a pale intermediate,
distinct from _egerides_ but approaching it. In Roumania all three
forms are recorded from various places: _egeria_ in the Dobrutscha; not
quite typical (presumably an intermediate) at Bukharest; intermediate
in various mountainous localities as well as in Macedonia and Dalmatia;
but _egerides_ in Azuga at about 3,000 feet.[10] Hungary has the true
_egerides_ also. (Cf. Caradja, _Deut. Ent. Zt._, IX, p. 58.) Mathew
records the same from Gallipoli (_E. M. M._, 1881, p. 95). Staudinger
does not distinguish the intermediates from the northern, but he gives
"_egerides_" for Armenia and Fergana (Central Asia). As against the mere
proximity of a great mountain chain being the influence which keeps
the Riviera population intermediate may be mentioned the fact that the
northern foothills of the Pyrenees have the pure southern type, and the
climate of Cambo must surely be far cooler than that of Nice. The exact
locality of the Greek specimens is not given, but there can be no part
of Greece which is not much hotter in summer than Brittany, or Calvados,
which have the intermediate, not the English type.
In face of these facts it can scarcely be maintained that average
temperature is the efficient cause of the particular tone of colour
which the butterfly shows in a given region. Nevertheless it is clear
that climate counts for much in determining the distribution. It is
noticeable that though the pale _egerides_ can be established in a
warm climate we never find _egeria_ in cold climates, and even the
intermediate is not found in places that have a hard winter. I suspect
that the distribution of the broods through the year and the condition
of the animal at the onset of hard frost are features which really
determine whether a strain can live in a particular place or not.
Though the truth of the suggestion cannot be tested by experiments in
captivity, which at once introduce disturbances, I incline to the idea
that _egeria_ has not got the right periodicity for northern climates.
If it could arrange its life so that the population consisted either
of young larvae, or perhaps of thoroughly formed pupae[11] at the
onset of winter, it might, for any obvious reason to the contrary,
be able to live in England. It is irregularly "polyvoltine," as the
silk-worm breeders say, and as soon as a little warmth encourages it,
a new generation starts into being, which if the frost comes at an
untimely moment, is immediately destroyed. Many species are continually
throwing off individuals which feed up fast[12] and emerge at once if
the temperature permits, and I imagine a species of Satyrid wholly
or largely represented by such individuals could scarcely survive in
a country which had a hard winter. For such a climate some definite
periodicity in the appearance of the broods may well be indispensable.
But assuming that _egeria_ is cut off from cold climates for such a
reason, there is nothing yet to connect these habits with the fulvous
colour, and until breeding can be carried out on a satisfactory scale
there is no more to be said.
From time to time records appear of individual specimens more or
less fulvous being caught in southern England, especially in the
New Forest.[13] It would be interesting to know what offspring such
individuals might produce. From the evidence now given some notion
both of the strength and the weakness of the case considered as one of
continuous climatic variation can be formed. I know no other equally
satisfactory. Whether or not definite mixture of the intermediates
with either of the extremes will be proved to occur, the case differs
materially from those considered in the last chapter in the fact that
at all events there is no general overlapping of forms. In a species so
little given to wandering, overlapping could indeed scarcely be expected
to occur. It is this circumstance which makes the species preeminently
suitable as a subject for the study of climatic influences, and I trust
that entomologists with the right opportunities may be disposed to
explore the facts further.
Just as many species, like _egeria_, have varieties which can be
regarded as adapted to northern and southern regions, so there are
also several which have lowland and Alpine forms quite distinct from
each other. Every such case presents an example of the problem we
have been considering. As the collector passes from the plains to the
Alpine region, how will he find the transition from one form to the
other effected? Does the lowland form give place to the Alpine form
suddenly, with a region in which the two are mixed, or will he find a
zone inhabited by an intermediate population? I have spent a good deal
of time examining the facts in the case of _Pieris napi_ and its Alpine
female variety _bryoniae_, and though there are many complications which
still have to be cleared up, no doubt is possible as to the main lines
of the answer. If in any valley in the Alps inhabited by both _napi_ and
_bryoniae_ the collector catches every specimen he can, beginning at
the bottom and working up to 7,000 feet, he will at first get nothing
but _napi_. At about 2,500 feet, he may catch an occasional _bryoniae_
flying with the _napi_. After 3,000 feet _napi_ usually ceases, and
only _bryoniae_ are found. As an exception a colony of _napi_ may be
met with at much greater heights. I once found them in numbers at about
6,000 feet.[14] Not only were they free from any trace of modification
in the direction of _bryoniae_, but they were of the thoroughly southern
type of _napi_, being a late brood of that large and very pale kind
(_meridionalis_) almost destitute both of dark veining above and of
green veining below, which are common on the shores of Lago Maggiore
and in other hot southern localities. Not far off at the same level
were typical _bryoniae_ in fair abundance. Occasionally an intermediate
may be met with. I have taken a few, for example, at Macugnaga and at
Fobello. These, however, in my experience are rarities in the Alps.
Fleck[15] gives notes on the distribution in Roumania which shows the
same state of things. The lowland form is not transformed though found
at great heights, and at Azuga (nearly 3,000 feet) _bryoniae_ occurs
with only occasional "_flavescens_," viz., intermediates of the second
brood.
If this were all the evidence we should be satisfied that the lowland
and Alpine types keep practically distinct, overlapping occasionally,
but rarely interbreeding. The problem would remain, how is the
distinctness of the two types maintained in the region of overlapping?
Nowadays, I suppose, we should incline to answer this question by
reference to segregation, and perhaps by an appeal to selective mating.
The suggestion that segregation does take place is certainly true to
some extent. There are, however, difficulties in the way, and the whole
subject is one of great complexity. My own experiments were made in
pre-Mendelian times and were not arranged with the simplicity which we
now know to be essential. The results are neither extensive enough nor
clear enough to settle the many collateral questions which have to be
considered, and the work ought to be done again. Nevertheless, some
notes of the observations may have a suggestive value.
When I began, I did not sufficiently appreciate that the "_napi_" group,
omitting the North American forms, and the Asiatic representatives, has
at least three chief types in western Europe. The differences we have
to deal with are manifested by the females only, so in this account
particulars as to the males are omitted for the most part. These are (1)
our own British _napi_; (2) the form found in the south, from the Loire
downwards, and in the Italian Alps, which I think may be spoken of as
_meridionalis_; (3) _bryoniae_, which is a form clearly recognizable in
the _female_ only, and is found only in the arctic regions and in the
Alps above 2,500 feet. The first two have several broods, two, three,
or more, according to opportunity, and the first brood is different
from the later ones. In _napi_ the markings on the upper surface are
a dark grey but in _meridionalis_ they are a pale silvery grey and
much less extensive. In the later broods of _napi_ there is much less
general irroration of the veins, and the spots stand out as more defined
and blacker. These differences vary greatly in degree of emphasis. In
_meridionalis_ the later broods are entirely different from the first.
Instead of having silvery markings they have the ground colour quite
white, with the spots large and a full black. On the under side of the
hind wings the usual green veins are almost absent, and I have seen
individuals which could scarcely be distinguished from _rapae_. To these
later broods the term _napaeae_ is sometimes applied, but I here use
_meridionalis_ for the southern race in general as applicable to all
broods.
The female _bryoniae_ is totally unlike the others. The ground colour
is a full yellow, and each nervure is thickly irrorated with a brown
pigment often spreading so far as to hide the ground almost entirely
in the fore-wings. The males corresponding with these females are not
certainly distinguishable from those of our own _napi_. Both sexes have
the green veining of the underside of the hind wing fully developed,
rather more than is usual in the lowland races, but this is not really
diagnostic of the variety. The first serious difficulty arises in regard
to the second brood of _bryoniae_. It is stated that there is only one
brood,[16] but I feel fairly sure that a second brood is sometimes
produced, and that the females with a yellow ground and diminished
irroration of the veins, not very uncommon in the Italian Alps in July
to August, are generally representatives of it. Such insects would of
course be classed with _bryoniae_ in collections.
My experiments began with eggs of true _bryoniae_ females caught at
about 2,500 feet early in July. These emerged in August-September as
intermediates with yellow ground and about half as much black on the
upper surface as _bryoniae_. They are exactly like the intermediates
usually found in nature and in the light of later experience I regard
them as natural F_{1} forms, and I think the mothers had been fertilised
by _napi_ males, though I admit that in view of the rarity of natural
intermediates there is a difficulty in this suggestion. Three of these
females were mated with males raised from thorough _meridionalis_
females, and three families were produced. Two of them showed distinct
evidence of segregation, some being yellow and some white with various
intergrades, some being no blacker than _meridionalis_ and some ranging
up to a dark intermediate type. Part emerged in the same autumn; and
part overwintered, emerging as the spring _meridionalis_ or as the
peculiar type which I afterwards learnt to know as the spring F_{1}
form. The distinctions were fairly sharp between the several forms. But
the offspring of the third female gave a series practically continuous
from _meridionalis_ to the F_{1} type. The work of subsequent years
gave results similarly irregular which could only be described
adequately at great length. The outcome may however be summed up in
the statement that there is evidence that both the yellow ground and
the dark veining are due to factors, but that there are several of
these and that imperfect segregation is not uncommon, producing various
reduction-stages. The yellow ground may be due to one factor, and the
several shades may be the result of irregularities in dominance, but
the black markings when fully developed cannot I think be the result of
less than three factors, one for the basal darkening, one for general
irroration, and one for the margins. Probably also the enlargement of
the spots is produced by a fourth factor.
There was not, in my experience any great difficulty in getting the
various forms to pair in captivity. Some attempts were made to see
whether individuals of either type selected mates of their own type in
preference to those of the other, but the results were inconclusive.
There were some indications of such a preference; though, from
the impossibility of judging how much of this may be due to other
circumstances, I could not come to a positive conclusion on the rather
meagre evidence.
Recently Schima[17] has given a careful and detailed account of all
the forms found in Lower Austria which he enumerates under 14 distinct
varietal names. He gives full references to previous accounts,
especially to the beautiful plates lately published by Roger Verity.[18]
Examination of these and of my own specimens strongly suggests that
the several forms are due to the recombination of the factors I have
named. Among those which I have bred are representatives of most if not
all the types enumerated by Schima in addition to other curious forms.
For example I have _bryoniae_ markings on a ground practically white;
the dark veins with spots almost obsolete; _meridionalis_ on a yellow
ground; the intermediate amount of black on a white ground, etc. The
last-named may occur wild and I have one from Macugnaga as well as one
given me by Mr. F. Gayner from Lulea (Lapmark).
To obtain really exact knowledge of the number of factors and
their properties it would be necessary to repeat the work. After
the beginning, I made a mistake in using British _napi_ instead of
_meridionalis_ and the results were much confused thereby. The contrast
between _meridionalis_ and the various dark forms is much greater and
classification of the types would have been therefore easier. The
British form is presumably _meridionalis_ plus the factor for the basal
pigmentation. The problem is greatly complicated by the differentiation
of the seasonal forms. The first point to be determined is whether
_bryoniae_ is capable of producing a second brood when it is thoroughly
pure-bred, and whether such a second brood is, as I suspect, normally
intermediate in character.
In the Alps generally there is no definitely intermediate population;
nor I believe, is any such population met with in the north where
the arctic _bryoniae_ meets _napi_, but as to this I have no precise
information. One curious fact, however, must be mentioned, namely that
there is a population that can probably be so described with fairness
established at Mödling near Vienna. This is not in any sense an Alpine
locality, and does not, as I am told, differ in any obvious way from
the other suburbs of Vienna. Dr. H. Przibram was so good as to send me
a set taken at this place, representing a second brood, and they were
decidedly heterogeneous, ranging from an intermediate form such as
_bryoniae_ fertilised by _napi_ usually produces, to a light yellowish
second-brood type with little dark pigment. There are also two actual
_bryoniae_. Whether true _napi_ also occur there I do not know, but I
have no doubt they do. It would be well worth while to investigate the
Mödling population statistically, and to breed from the intermediates
which might not impossibly prove to be heterozygotes. There are also
records of such intermediates being occasionally found in some parts
of Ireland, in the north of Scotland, and in south Wales,[19] but I do
not know of any regular colony of these forms. We can scarcely avoid
the inference that one or more of the factors which make up _bryoniae_
may be carried by these intermediates. It is not clear why their
interbreeding does not produce actual _bryoniae_ occasionally. If this
occurred, the probability is that the fact would be known to collectors,
at least in the British localities. The absence of true _bryoniae_ must,
I think, be taken to mean that some essential factor is absent from
these intermediates.
To sum up the evidence, the facts that are clear may be thus enumerated:
1. _Napi_ and _bryoniae_, or in the Italian Alps,
_napaeae_ and _bryoniae_ frequently meet each other.
2. They cross without difficulty, producing fertile offspring.
3. But in the levels at which they overlap there is no
intermediate population, and only occasional intermediate
individuals.
4. In certain parts of the distribution of _napi_ similar
intermediates sometimes occur, and at one place (Mödling) they
are so frequent as apparently to constitute a colony.
5. As to the genetic relations of the two forms there is no
complete certainty. Indications of segregation have been observed
in some cases, but there are several factors concerned and they
are liable to some disintegration.
Another form in which I tried to investigate the same problem is
_Coenonympha arcania_, which has one Alpine form known as _darwiniana_,
and another, _satyrion_. In calling _satyrion_ a form of _arcania_ I
follow Staudinger and other authorities, but I have never been quite
satisfied that it should be so regarded. The differences between
_arcania_ and _darwiniana_ are essentially differences of degree; _C.
arcania_ occurs in places where there is cover, and reaches up the
valleys usually as high as the mixed woods of deciduous trees, which
is about 2,500 feet. The variety _darwiniana_, on the contrary, is an
insect of treeless hillsides, and I regard it as a dwarf and possibly
a stunted form. It would not greatly surprise me to find that with
the application of good conditions _arcania_ could be raised from
_darwiniana_ eggs, or that if _arcania_ larvae were starved they might
give rise to _darwiniana_ butterflies. I have been unsuccessful in
trying to rear the species, having lost the larvae by disease. Usually
one does not catch _arcania_ and _darwiniana_ on the same ground, and
as _Festuca ovina_--a typically hill-side grass--is a common food-plant
of _darwiniana_ there can be little doubt that _arcania_ feeds on
some other grass, probably woodland species. Colonies of _arcania_ of
varying size and brightness are commonly found, and though a sample of
_arcania_, finely grown, from a warm Italian wood, presents a striking
contrast with _darwiniana_ from an Alpine pasture, one certainly may get
samples which fill all the gradations. Generally the sample from a given
locality is fairly homogeneous.
Of _satyrion_ I have little personal experience. I only twice found
it, namely at Zinal, and at Hallstatt in Austria, but it occurs at
Zermatt, Arolla, and in several Swiss localities above 5,000 feet, and
I understand that it is the typical Alpine form in the Engadine. With
its darkened colour and reduced size it might well be expected to be a
still further stunted form of _darwiniana_. Yet I have never found the
one succeed to the other at the higher levels. If _darwiniana_ appears
when Alpine conditions are reached in a valley it will be met with
up to the highest level at which such butterflies live. Tutt was of
opinion that _satyrion_ is a distinct species.[20] I once, at the top
of the Vorderrheinthal caught a sample of _darwiniana_ a few of which
(males) were so dark and had the eye spots so poorly developed that they
looked like transitions to _satyrion_. Otherwise I never found any such
transitional forms and they are certainly exceptional. There is further
a record[21] of _satyrion_ having been taken flying with _arcania_.
This was near Susa, at about 2,000 feet I infer. Mr. H. E. Page has
similar specimens from Caud and from St. Anton (Arlberg). The females,
however, both of mine and of Mr. Page's samples are a pale brown, quite
unlike the females both of _arcania_ and of the dark Zinal _satyrion_.
The difficulty thus raised has not I think yet been considered by the
authorities, and it is possible that the Alpine forms of _arcania_ are
in reality three, not two.
The evidence taken together suggests, I think, that _darwiniana_ is
related to _arcania_ much as so many of the Alpine varieties of plants
are to the well-developed individuals of the lower levels. I do not
anticipate that factorial differences will be found in these insects,
and it is by no means impossible that the distinctions between them
are the direct consequences of altered conditions. The relations of
_arcania_ to _satyrion_ are more doubtful, and in that case a factorial
difference may at least be suspected.
The species of the genus _Setina_ have Alpine forms which agree in
possessing a characteristic extension of the black pigment to form
radiating junctions between the spots on the wings. Speyer, who
discussed the interrelations of these forms in detail,[22] lays stress
on the absence of genuine transitional forms between _aurita_ and the
variety _ramosa_. Both are mountain insects but _ramosa_ extends to
levels higher than that at which _aurita_ ceases, which is about 4,000
feet. The two forms are often found flying together. Speyer says that
his brother searched diligently for transitional forms at the level of
overlapping, but found none, so that at least they may be regarded as
rare. The variety _ramosa_ is not infrequent at much lower levels (_e.
g._, Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high
as the permanent snows. In the British Museum collection, however, I
have seen several that I should regard as transitional. Speyer perhaps
would have classed as _ramosa_ all in which the spots of the central
field were united, and it is by no means unlikely that breeding would
prove such individuals to be heterozygous.[23]
There can scarcely be a doubt that the distinction between _aurita_ and
_ramosa_ is factorial, the radiate _ramosa_ probably having the factor
for striping. In support of this view may be mentioned the observation
of Boisduval,[24] respecting a gynandromorphous individual, which was
_aurita_ male on one side, and _ramosa_ female on the other. Speyer
makes another excellent comment. He points out that the simple notion
that the radiation is a mere extension of pigmentation consequent on
the climate of the higher levels, will not fit the facts very easily,
because the size of the spots varies greatly in _aurita_ itself at any
level, and lowland specimens may actually have more black confined
to the spots alone than some _ramosa_ possess on spots and lines
combined.[25]
The two Salamanders, _S. maculosa_ and its Alpine form _atra_, might
not improbably furnish evidence bearing on the same problem. The two
are of course very distinct, not merely in colour (_maculosa_ being
spotted with yellow or orange while _atra_ is entirely black) but also
in the mode of reproduction, a feature to which reference will be made
in the next chapter. I cannot, however, find any evidence as to the
overlapping of the two forms. _S. atra_ occurs from about 3,000 feet
or somewhat less, and reaches great elevations in the Eastern Alps,
but I do not know if the two forms ever occur in the same localities.
Leydig,[26] Boulenger,[27] and most modern authorities regard the two
types as distinct species, but they are in any case closely allied, and
it would be of interest to have exact knowledge of their geographical
delimitations.
The reader who has considered the cases adduced will appreciate the
difficulties which must be faced in any attempt to account for the
facts in a rational way. As always in a problem of Evolution, two
separate questions have to be answered. First how did the form under
consideration come into existence, and secondly, how did it succeed
in maintaining itself so as to become a race? The evidence from the
local forms, though very far from giving complete answers to either of
these questions definitely refutes the popular notion that a new race
comes into existence by transformation of an older race. If a gradual
mass-transformation of this kind took place we should certainly expect
that when two types, nearly allied and capable of interbreeding, overlap
each other in their geographical distribution, a normally intermediate
population would exist. If each type can maintain itself, and if
each came into existence by gradual transformation, then there must
have been an intermediate capable of existing and maintaining itself
as a population; and if this had ever been, surely in the region of
overlapping, that intermediate population should continue. Especially
should such a population be found when the two extreme types are
adaptational forms and the region of overlap is a region of intermediate
conditions. But of the examples we have examined there is only one, that
of _Pararge egeria_ and _egerides_, which can at all be so interpreted,
and even in that case it is not impossible that more minute observation
would reveal discontinuity between the extremes and the admittedly
normal intermediate population. Granting provisionally however that
this example, as it stands, is consistent with the conventional theory
of evolution, I know not where we should look for another case equally
good. When the distinctions are produced by direct influence of
conditions operating during the lifetime of the individuals, examples of
intermediate populations occupying the areas of intermediate conditions
can no doubt be produced. Many turf-like Alpine plants, for instance,
if protected from exposure and properly nourished can grow as large as
those of the same species found in the valleys, and in the case of such
quantitative effects, intermediate conditions can doubtless produce
intermediate characters.
Even these examples however are not very abundant, and often the
intermediate locality has not a form intermediate between those of
the two extreme localities, but some third form distinct from either.
This is the case for instance in the fauna of brackish waters. We are
taught to believe that the fresh water fauna was evolved from the
marine fauna, which it well may have been; but as students of Crustacea
and Mollusca know familiarly, the brackish water forms are not as a
rule intermediates between fresh water species and sea species, but
more usually they are special forms belonging to the brackish waters,
with the peculiar property that they can tolerate a great range of
conditions, and live without ostensible variation in waters of most
various compositions and densities, which very few marine or fresh water
species are able to do.
Sometimes the distinction between local races, as in _Rhamphocoelus
passerinii_ and _icteronotus_ may be regarded with confidence as due to
one simple Mendelian factor possessed by one race and absent from the
other, but I think, more often, as in _Colaptes_ or in the varieties
of _Pieris napi_, the existence of several distinct factors is to
be inferred. As we have seen, the races of _Colaptes_ show almost
beyond doubt that in different areas at least three distinct factorial
combinations can be perpetuated as races.
In the distribution of variability we find, I think, some hint as to
the steps by which the phenomena under consideration have come to their
present stage, and I am disposed to regard the facts so well attested
in the case of our own melanic moths as a true indication of the
process. Following this indication we should regard the change in the
character of a population as beginning sporadically, by the appearance
of varying individuals, possibly only one varying individual, in, it may
be, one place only. As to _why_ a variety should increase in numbers
we have nothing but mere speculation to offer, and for the present we
must simply recognise the fact that it may. That such survival and
replacement may reasonably be taken as an indication that the replacing
race has some superior power of holding its own I am quite disposed to
admit. Nevertheless it seems in the highest degree unlikely that the
outward and perceptible character or characters which we recognise as
differentiating the race should be the actual features which contribute
effectively to that result.
In discussions of geographical distribution in relation to problems of
origin it is generally said that very nearly allied species usually
occupy distinct areas, while other competent observers state the exact
contrary. Lately, for example, Dr. R. G. Leavitt[28] has published an
important collection of evidence upholding the latter proposition, taken
chiefly from the botanical side, showing how in numerous genera two or
more closely allied species coexist, frequently without intermediates,
in the same localities, and may even be thus found in company throughout
their distribution. The difference of opinion evidently arises from a
confusion as to the sense in which the term "species" is understood and
applied. Leavitt, for example, is avowedly following Jordan and, among
moderns, Sargent, in applying a close analysis, and denoting as species
all forms which are distinct and breed true. Against this use of the
term I know no valid objection[29] but it must be obvious that if others
follow a different practice confusion may result when observations are
summarised in general statements. We will consider this subject again
in another place, but here it may be sufficient to say that there can
scarcely now be a doubt that numbers of these associated species, such
as Jordan discriminated, represent various combinations of the presence
and absence of Mendelian factors. This does not in any way weaken the
argument which Leavitt founds upon the facts, namely, that the observed
distribution of these forms is consistent with the supposition of an
evolution largely discontinuous.
On the other hand, those who have come to the opinion that nearly
allied species generally occupy distinct ground are presumably more
impressed by the characters differentiating the geographically distinct
or adaptational races, seeing that genuine intermediates between
them are less commonly found. Those geographical races may no doubt
contain various differentiated forms; but when all live together,
occasional intermediates are usually to be found even in the case of
characters habitually segregating. These segregating forms Jordan would
certainly have determined as species, and it must be conceded that no
physiological definition has yet been drawn which consistently excludes
them.
FOOTNOTES:
[1] Often referred to by older writers as _Meone_, Esper's name.
[2] There are also two distinct island forms, unlike the European,
_Xiphia_ of Madeira, and a smaller variety, _Xiphioides_ of Canary. See
especially, Baker, G. T., _Trans. Ent. Soc. London_, 1891, p. 292.
[3] Speyer, Adolf, and August. _Verbreitung der Schmetterlinge_, 1858,
I, p. 217.
[4] _Lepid. Comparée_, fsc. III, p. 372.
[5] Mr. Rowland-Brown has called my attention to a statement by Dr.
Vaillantin (_Petites Nouv. Ent._, II, 235) that in Indre-et-Cher the
first brood is of the northern type and the second of the southern. My
experience is that in captivity these distinctions do not occur, and I
have true _egeria_ as first brood from Vienne and as the late brood from
the Landes. I never collected in Indre-et-Cher.
[6] I have since seen true _egeria_ from Ferrol in the extreme
northwest, which was in Mr. Tutt's collection.
[7] Mr. G. Wheeler kindly showed me a series identical with this type,
from Guernsey, and others from near Laon.
[8] _Ent. Rec._, V, 1894, p. 134.
[9] Mr. Wheeler has some pale but rather worn specimens from the Rhone
Valley at Vernayaz.
[10] See Fleck, E., Die Macrolep. Rumäniens, _Bul. Soc. Sciinte_, VIII,
1899, p. 720.
[11] My experience agrees with that of Mr. H. Williams (_Ent. Rec._,
VIII, 1896, p. 181) that pupae, well-formed, can stand considerable
frost; but I used to find that half-grown larvae usually died if
unprotected, and I believe that larvae which attempted to pupate in warm
autumn weather and then got caught by frosts, always died. Small larvae
which can creep into shelter at the bottom of the plants survived, and I
expect that in the north the winter is usually passed in that state (see
also Merrifield, F., _Ent. Rec._, VIII, 1896, p. 168, and Carpenter, J.
H., _ibid._).
[12] Some most unlikely species do this. I once had a larva of
_Parnassius delius_, found at about 5,500 feet, which emerged late
in the autumn (in October I believe), a season at which it must have
perished in its own country.
[13] See, for examples, Barrett, G. C., _Lepidoptera of the Brit.
Islands_, I, 1893, p. 229; also Grover, W., _Ent. Rec._, IX, 1897, p.
314; Williams, H., _Proc. Ent. Soc._, 1898, who reared several specimens
from the New Forest which would pass for Bretons, though the rest of the
family were true _egerides_.
[14] Above the Tosa falls.
[15] _Bul. Soc. Sciinte_, VIII, 1899, p. 691.
[16] The fact that Weismann by heating pupæ obtained only one autumn
specimen seems to me to show rather that a second brood can be produced
than that it cannot, which is the inference usually drawn.
[17] Schima, K., _Verh. Zool. bot. Ges. Wien_, LX, 1910, p. 268.
[18] _Rhopalocera Palaearctica_, Florence, 1905-11, especially Pl. XXXII.
[19] See figures in Barrett, G. C., _Lepidoptera of Brit. Islands_, I,
pt. 3, p. 25.
[20] Tutt, J. W., _Ent. Rec._, XVIII, 1905, p. 5. In the same place he
states that on the Mendel Pass _arcania_ "runs into" _darwiniana_ and
that in the Tyrolean localities the transition is especially evident.
Wheeler (_ibid._, XIII, 1901, p. 121) expresses the contrary opinion,
that _satyrion_ does grade to _arcania_.
[21] H. Rowland-Brown, _Ent. Rec._, XI, 1899, p. 293.
[22] Speyer, Stettiner, _Ent. Ztg._, XXXI, 1870, p. 63.
[23] In regard to the closely analogous case of _Spilosoma lubricipeda_,
Standfuss makes a similar statement. He bred the type on a large scale
with the radiate form which he calls _intermedia_, and says that in four
years of miscellaneous crossing he never obtained really transitional
forms. Nevertheless after examining large series, especially those of
Mr. W. H. B. Fletcher, I came to the conclusion that several might be
so classed, but I am quite prepared to find that such specimens are
heterozygous. (See Standfuss, _Handb. d. Gross-Schmet._, 1896, p. 307.)
It is by no means unlikely that various dark forms of _lubricipeda_
correspond with a progressive series of factorial additions. Many of
the stages have been named, and of these the most definite are the
_intermedia_ of Standfuss (probably = _eboraci_ of Tugwell) and the very
dark _Zatima_ of Heligoland, in which only the thorax, the nervures
and a small field in the fore-wings remain yellow. A form was bred
by Deschange from _Zatima_ in which even the field in the forewing
is obliterated. The exact circumstances in which _Zatima_ occurs in
Heligoland would be worthy of special investigation, for the normal
_lubricipeda_ is also found on the island. For references as to the
British occurrences see especially, Hewett, W., _Naturalist_, 1894, p.
353. As to _Zatima_ see especially Krancher, _Soc. Ent._, II, 1887-8, p.
26. I am indebted to Dr. Hartlaub for information as to the Heligoland
types.
[24] Boisduval, _Bull. Soc. Ent. Fr._, III, 1834, p. 5.
[25] The systematics of _Setina_ have been much controverted, but no one
I believe doubts that _aurita_ and _ramosa_ are forms of one species.
See also Chapman, A. T., _Ent. Rec._, XIII, 1901, p. 139.
[26] _Arch. Naturg._, 33, 1867, p. 116.
[27] _Brit. Mus. Cat., Batrachia Gradientia_, 1882.
[28] The Geographical Distribution of nearly related Species. _Amer.
Nat._, XLI. 1907, p. 207.
[29] See later, p. 242.