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ON THE VARIATION OF SPECIES
WITH ESPECIAL REFERENCE TO THE INSECTA;
FOLLOWED BY AN INQUIRY INTO THE NATURE OF GENERA.
BY
T. VERNON WOLLASTON, M.A., F.L.S.
"No compound of this earthly ball
Is like another, all in all."
TENNYSON.
LONDON:
JOHN VAN VOORST, PATERNOSTER ROW.
1856.
"I do not enter so far into the province of the logicians as to take
notice of the difference there is between the _analytic_ and
_synthetic_ methods of coming at truth, or proving it;--whether it is
better to begin the disquisition from the subject, or from the
attribute. If by the use of _proper media_ anything can be showed to
be, or not to be, I care not from what term the demonstration or
argument takes its rise. Either way propositions may beget their like,
and more truth be brought into the world."--_Religion of Nature
Delineated_, p. 45 (A.D. 1722).
PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.
TO CHARLES DARWIN, ESQ., M.A., V.P.R.S.,
Whose researches, in various parts of the world, have added so much to
our knowledge of Zoological geography, this short Treatise is
dedicated.
PREFACE.
To make a dry subject entertaining, is impossible; but to render it,
at any rate, readable, has been my endeavour in the following pages.
How far I have succeeded in the experiment, it is not for me to
decide.
It having been suggested, by several of my friends, that it might be
desirable to bring together into a small compass some of the evidence
on Insect variation (with reference to external disturbing causes)
which my researches in the Madeira Islands have supplied me with, I
have been encouraged to do so: and I have added numerous conclusions
from other data also, which have from time to time fallen in my
way,--so as to confer on the volume a more practical interest, for
the general naturalist.
One of my main objects, however, has been to call attention to the
fact, that the Annulosa have not been hitherto sufficiently
considered, in the great questions arising out of the distribution of
animals and plants; hoping that, by so doing, some few of our British
entomologists, who have not looked into this branch of their science,
may be induced to enlist themselves in the cause of Insect geography.
If such a result be brought about; or if I be fortunate enough to open
for discussion any of the topics which have been touched upon, and so
lead to a more perfect solution of the problems which I have attempted
to explain, I shall consider myself more than repaid.
10 Hereford Street, Park Lane, London.
May 10th, 1856.
CONTENTS.
CHAPTER I.
Introductory Remarks
CHAPTER II.
Fact of Variation
As a matter of experience
As probable from analogy
CHAPTER III.
Causes of Variation
Sec. 1. Climatal causes generally (whether dependent
upon latitude or upon altitude)
Sec. 2. Temporary heat or cold, of an unusual degree
Sec. 3. Nature of the country, and of the soil
Sec. 4. Isolation; and exposure to a stormy atmosphere
CHAPTER IV.
Organs and Characters of Variation
CHAPTER V.
Geological Reflections
CHAPTER VI.
The Generic Theory
CHAPTER VII.
Conclusion
CORRIGENDUM.
Page 90, for _Pecteropus Maderensis_ read _Pecteropus rostratus_.
SPECIFIC VARIATION IN THE INSECTA.
CHAPTER I.
INTRODUCTORY REMARKS.
A very small amount of information gained by the student in the field
of Nature is sufficient to kindle the desire to increase it. The more
we know, the more we are anxious to know; though the less we seem to
know. It is one of the distinctive privileges of the naturalist that
he has to labour in a mine which is inexhaustible: the deeper he digs
beneath the surface, the richer is the vein for excavation, and the
more interesting are the facts which he brings successively to light.
Dive he ever so deep, Truth, "at the bottom of the well," is assuredly
present, under some form or other, to reward him still; nor will she
even for once elude his grasp, provided he be content to receive her
as she is, instead of endeavouring to mould her to his preconceived
ideas of what she ought to be. In these times of patient research,
when the microscope is disclosing, day by day, fresh wonders to our
view, and new lines of speculation are springing out, as it were
spontaneously, from the regions of thought, it is remarkable that many
of the commoner questions relating to the members of the external
world around us have remained comparatively unsolved; nor indeed have
some of them ever been discussed at all, except in a desultory manner
and with insufficient data to reason from. Foremost amongst these,
numerous problems affecting the distinction between "varieties" and
"species" (as usually accepted) of the animal kingdom stand
pre-eminent,--especially in the Annulose Orders, in which those
distinctions are less easy, _a priori_, to pronounce upon.
The descriptive naturalist, whose primary object it is to register
what he sees (apart from the obscurer phaenomena which come within the
province of the more philosophical inquirer), can have scarcely failed
to remark the variation to which certain insects are at times liable
from the external agencies to which they have been exposed: and yet,
in spite of this, it is but too true that even physiologists have
frequently shunned the investigation of the _circumstances_ on which
such variations do manifestly in a great measure depend, as though
they were in no degree accountable for the changes in question, and
did not indeed so much as exist except in theory. In the following
pages I purpose, _inter alia_, to throw out a few general hints;
first, on the fact of aberration, as a mere matter of experience;
and, secondly, on some of the _causes_ to which the physiologist
would, in many instances, endeavour to refer it.
The _former_ of these considerations (namely, the _fact_ of specific
instability as ordinarily noticed) nobody will be inclined to dispute:
and yet it is abundantly evident that it cannot be taken into account,
at any rate satisfactorily, without involving the _latter_ also,--it
being scarcely possible to attach the proper value to an effect
without first investigating its cause. The importance of assigning its
legitimate weight (and that only) to a variety, is perhaps the most
difficult task which the natural historian has to accomplish; since on
it depends the acknowledgment of the specific identity of one object
with another,--whilst, to draw the line of separation between
varieties and species is indeed a Gordian knot which generations have
proved inadequate to untie. Now it is not the object of this
publication to attempt to throw positively new light upon a subject
which has ever been one of the main stumbling-blocks in the lower
sciences, and which is perhaps destined to be so to the end; still
less would I wish to imply that the causes of variation _are_
altogether overlooked in these days of accurate inquiry,--when
thousands are accumulating data, in all parts of Europe, destined to
be wielded by the master's hand whensoever the harvest-time shall have
arrived: but I do, nevertheless, believe that there exists a growing
tendency, especially in some portions of the Continent, to regard
every difference (if at all permanent) as a specific one; and hence I
gather the information that a reviewal of our first principles is
occasionally necessary, if we would not restrict (however gradual and
imperceptibly) that legitimate freedom which Nature has had chalked
out for her to sport in, or strive to impose laws of limitation in one
department which we do not admit to be coercive in another.
Perhaps, however, before entering on the subject-matter of this
treatise, my definition of the terms "species" and "variety,"--so far
at least as such is practicable,--will be expected of me. I may state,
therefore, that I consider the _former_ to involve that ideal
_relationship amongst all its members_ which the descent from a common
parent can alone convey: whilst the _latter_ should be restricted,
unless I am mistaken, to those various aberrations from their peculiar
type which are sufficiently constant and isolated in their general
character to _appear_, at first sight, to be distinct from it.
The _first_ of these enunciations, it will be perceived, takes for
granted the acceptance of a dogma which I am fully aware is open to
much controversy and doubt,--namely, that of "specific centres of
creation." Without, therefore, examining the evidences of that theory
which would be out of place in these pages (and which has been so ably
done already by the late Professor Edward Forbes), I would merely
suggest that the admission of it is almost necessary, in order to
convey to our minds any definite notion of the word "species" at all:
and that, hence, whilst I would not wish to reject the hypothesis as
involving an absurdity (which I believe to be the exact opposite of
the truth), I would, in the present state of our knowledge, desire
rather to regard it as a _postulate, assumed to illustrate the
doctrine of species_, than as a problem capable of satisfactory
demonstration.
The _second_ of the above definitions may likewise require briefly
commenting upon; for I have frequently heard it asserted that
everything is to be regarded as a "variety" which has wandered in the
smallest degree from its normal state. Now this I contend is
essentially an error; for a "variety," to be technically such, must
have in it the _prima-facie_ elements of stability,--and to an extent
moreover that, without the intermediate links (which, although rarer
than the variety itself, _must nevertheless exist_) to connect it with
its parent stock, its condition is such that it might be registered as
specifically distinct therefrom. Thus, to take an example for
illustration, there are many darkly coloured insects which, as every
entomologist knows, vary, by slow and regular gradations, into a
pallid hue, sometimes into almost white. It also most frequently
happens, in such instances, that the _extreme_ aberration is of more
common occurrence than the intermediate ones. Here then is a case in
point: there is but a _single_ variety involved, namely a pale
one,--the gradually progressive shades which imperceptibly affiliate
it with its type not being regarded in themselves as "varieties" at
all. If this indeed were not so, then would our position be far from
pleasant, since we should be compelled to record, as a variety,
_every_ separate degree of colour which could possibly be found
between the outer limits,--seeing that (increasing, as they did, in an
even ratio) no _one_ could be tabulated in preference to another.
This however is an example in which the rate of alteration (so far as
colour is concerned) is _equal_; and one therefore in which the
extreme end of the series can be alone singled out as _the_ aberration
to be specially noticed. It sometimes occurs that, between the two
extremes, there are several nuclei, or centres of radiation, to which
the name of varieties may be legitimately applied,--inasmuch as they
may possess a series of characters which do not, all, in combination,
progress evenly; and which consequently stand out as it were, to as
certain extent isolated, from the remainder.
As a corollary arising out of these remarks, it would seem to follow
that even small differences _should be regarded as specific ones_ so
long as the intermediate links have not been detected which may enable
us to refer them to their nearest types. In a general sense, I believe
that it would be proper to do so: nevertheless there are instances,
the results, for example, of isolation, in which _abrupt_
modifications may be _a priori_ looked for; and in which our judgment
must be regulated by our knowledge of the local circumstances which
may be reasonably presumed to have had some influence in producing
them. The consideration of these, however, and other kindred
questions, must be deferred to a subsequent chapter of this work.