a Worker. _Od_, oviduct. _A_, one ovary of _Myrmica lævinodis_ with
many ovarian-tubes, in each of which there is an almost ripe egg (_Ei_)
and a younger egg (_Ei´_). _B_, the ovaries of a Worker of _Lasius
fuliginosus_; each ovary has only one ovarian-tube, and no ripening
egg-cells. After Elizabeth Bickford.]
Against this proposition an observation by O. vom Rath has been
cited. According to it, three drone larvæ which had been accidentally
fed by the workers with royal food exhibited striking retrogressive
peculiarities in their sexual organs. The testes contained only
immature sperms (just before emergence from the pupa), and the
copulatory organ was entirely wanting. That a certain degree of fatty
degeneration of the testes should be caused by the 'unusual fattening'
is not surprising, but it seems to me very questionable whether the
absence of the copulatory organ can be referred to the abnormal
diet; it ought to be definitely decided, by the investigation of
numerous cases, whether some abnormal peculiarity in the constitution
of the germ-plasm in these eggs was not the true cause. Hitherto,
unfortunately, I have not been able to procure the fresh material
necessary to decide this point[15]. From all this it must be evident
that we are not justified in regarding either the absence of wings or
the degeneration of the ovaries as a direct result of the inferior
nourishment supplied to the workers in the larval state: but should
any one still have doubt on this point I may mention that, among our
indigenous ants, there are two species in which the workers are just as
large as the fertile females, and that in tropical America a species
(_Myrmecocystus megalocola_) occurs in which the workers are larger
than the true females; this must mean that they have received more food
than the females, though perhaps not the same mixture of food.
[15] Since completing my manuscript I note that the point was settled
three years ago, when Koshewnikow had the opportunity of investigating
drone-pupæ which were abnormally reared in royal cells, and therefore
fed with royal food. He found their sexual organs perfectly normal, and
agrees with me that the abnormalities in Vom Rath's case must have been
due to some other cause. (See the report by Von Adelung on the Russian
paper in _Zool. Centralblatt_, Sept. 10, 1901.)
From all the facts we have discussed we can confidently conclude that
the differences in structure, which distinguish the workers from the
true females, do not depend upon the influence, in the individual
lifetime, of a poorer diet, but upon variation in the primary
constituents of the germ; we must conceive of the germ-plasm of ants as
containing, in addition to male and female ids, special ids of workers,
in which the determinants of wing and ovary are degenerate in some
degree, while the determinants of other parts, such as the brain, are
more highly developed. The manner of feeding, however, and perhaps the
mingling with the food of a special secretion of the salivary glands,
acts as a stimulus which determines whether one kind of id or another
is to be liberated, that is, to become active and to enter on the path
of development.
A proof of this view is to be found, it seems to me, in the existence
of transition forms between workers and true females, which was first
brought to general knowledge by Forel. Perhaps it would be better to
call these 'mongrel forms' for their various parts do not maintain a
medium between the two types, but many parts follow the type of the
worker, and others that of the true female. Thus Forel twice found a
nest of the red wood-ant which contained a large number of these mixed
forms, all of which possessed the small head and large curved thorax of
the queen, but otherwise resembled the workers in size and appearance,
and also in the degeneration of the ovaries. Many of them were very
small, only 5 mm. in length, and had probably received very little
food, and, according to the theory of direct influence, these should
have been pure workers. That they possessed the head and thorax of a
queen is a proof that the characters of both forms of individual were
present in the germ-plasm as primary constituents, or indeed entire
ids. In normal circumstances only one kind of these ids would have
become active, either the worker-id or the queen-id, but in abnormal
circumstances they might both be liberated to activity simultaneously,
and then they would stamp one part of the body with the character of
a queen, another with that of a worker. Forel observed one of these
nests in two successive years, and both times found the mixed forms
in large numbers[16]. In the second year he found a great number of
newly-emerged individuals of this type. I have already inferred from
this observation that the mixed forms were probably in both years
the offspring of the same mother, and this may well have been the
case. My further conclusion, that the mixed forms must be due to some
abnormality in the constitution of the germ-plasm of the maternal eggs,
no longer appears to me so convincing as it did formerly, because, in
the interval, we have learnt, through that indefatigable investigator
of ants, Pater Wasmann, that there is another possible explanation of
these mixed forms; it, too, is based upon a hypothesis, but it is so
interesting that I must briefly outline it to you.
[16] There are different kinds of 'mixed forms' among ants, which may
owe their origin to a variety of conditions, as Forel, Wasmann, and
Emery have shown in detail.
Like Forel and myself, Pater Wasmann had supposed that the reason of
this kind of mixed form (the so-called pseudogynous worker) lay in an
abnormality of the constitution of the germ-plasm, but he now regards
it as the result of a change in the mode of rearing instituted by the
workers with respect to the constitutionally female or queen larvæ,
because there was a scarcity of workers. The hypothesis sounds very
daring, but it is well founded, at least in so far that there really
is a reason why a scarcity of females must occur at certain times in
some colonies of ants, and this might certainly determine the workers
in charge of the larvæ to feed females with worker food, so as to rear
them to render the necessary assistance.
This reason lies in the occasional presence of a parasitic beetle,
_Lomechusa strumosa_, whose larvæ, curiously enough, are cared for and
fed by the ants as though they were their own, and in return they eat
up the larvæ of the ants, often destroying them in large numbers.
Wasmann informs us that the parasitic larvæ grow up just at the time at
which the ants are rearing their workers, and it is these, therefore,
which fall victims to the Lomechusa-larvæ, and the result is that a
scarcity of young workers must soon make itself felt. The workers
seek to make this good by rearing as workers all the larvæ previously
destined for queens. But this only succeeds partially, because the
development towards true females has already begun; thus mixed forms
arise.
This explanation would be rather in the air if we did not know that,
among bees, such changes in the manner of rearing are by no means
uncommon. Indeed they occur regularly when the queen of a hive perishes
and no more 'female' eggs are in store; young worker larvæ are then fed
with royal food, and these develop into queens. There can thus be no
doubt that these insects have it in their power to liberate to activity
either the female ids or the worker ids by a specific mode of feeding,
and there is nothing contrary to reason in admitting the possibility
of an alternation of this influence in the course of development, for
something analogous occurs in regard to secondary sexual characters,
as, for instance, the appearance of male decorative colours in ducks
that have become sterile.
But this change in the mode of rearing bee-larvæ gives rise to pure
queens and not to mixed forms, and we must therefore regard it as
undecided whether Wasmann's explanation is correct in this case, and
whether an abnormality in the constitution of the germ-plasm may not be
the true cause of this or other kinds of mixed forms among ants. In any
case the 'Lomechusa hypothesis' rests upon the assumption of different
kinds of ids in the germ-plasm, as Pater Wasmann expressly states, and
the differences between the worker and queen-ants have their cause in
this, and not directly in the kind of larval food.
If there were not different ids corresponding to the different kinds
of individuals in the germ-plasm a kind of polymorphism might indeed
have arisen in the colony through differences in nutrition, but it
could not have been of the kind we now see--that is, a sharply defined
differentiation of persons, in adaptation to their different functions.
This presupposes elements in the germ which can vary slowly and
consistently in a definite direction without causing any change in the
rest of the germ.
This state of affairs gives to the phyletic evolution of the workers
a great theoretical significance, for it proves that positive as
well as negative variations of the most diverse parts of the body,
that simultaneous and correlative variations of many parts, can take
place in the course of the phylogeny, without the co-operation of
the Lamarckian factor. I have not hitherto laid any special emphasis
upon the degree of differences occurring between workers and queens;
but I must now add that this may far exceed the degree that we are
familiar with in our common indigenous ants, both in regard to
instinct and to bodily form. Even in the red Amazon ant of Western
Switzerland, _Polyergus rufescens_, we find quite a new instinct[17],
that of carrying off the pupæ of other species of ants, not to devour,
but to introduce them to their own nest and thus secure 'slaves.'
For these workers of a strange species, which emerge in a strange
nest, naturally regard the place of their birth as their home, and
do there what instinct impels them, and what they would have done in
the nest of their parents: they feed the larvæ, fetch food, collect
building material, and so on. The domestic activity of the workers
of the master-species thus becomes superfluous, and they have ceased
to exercise it, and have now entirely lost the power of caring for
their brood, searching for food, and keeping up the nest. They have
even forgotten how to take food themselves, because they are always
fed by the 'slaves.' Forel informs us--and I have myself repeated the
experiment--that _Polyergus_ workers, which are shut up with a drop
of honey on the floor of their prison, will leave it, their favourite
food, untouched, and finally starve, unless one of their 'slaves' be
shut up with them. As soon as this happens, and the slave perceives the
honey, it partakes of it, and then the 'mistress' comes and strokes the
'slave' with her antennæ to signify her desires, whereupon the 'slave'
proceeds to feed her from its own crop.
[17] 'New' in this sense, that the instinct is not exhibited by most
worker-ants, that it did not occur in the primaeval ancestors of modern
ants. It is, however, exhibited by a number of modern forms, and even
by some German species.
But while the _Polyergus_ workers have forgotten their domestic habits,
and have even ceased to be able to recognize their food, remarkable
changes have taken place in their jaws; these have lost the blunt teeth
on the inner margin, which, in other species, serve for masticating
the food, for seizing building material, and for other domestic
occupations, and have become sharp weapons, bent in the form of a
sabre, very well suited for piercing the head of an enemy, but also
well adapted for carrying off the pupæ, because they can seize them
without doing them any injury.
No one will doubt that the predatory expeditions of the Amazon ants,
and the slave-making habit, can only have developed after the habit
of living in large companies had long existed, and this case proves
that variations of instinct, as well as of bodily structure, can
take place even after the workers have long been sterile. The case
is the more instructive that it _seems_ as if it were due to the
transmission of a newly acquired and inherited habit of life, while
in point of fact these Amazon-workers can transmit nothing, because
they bear no offspring. But if old instincts can be lost, and new ones
acquired, when all possibility of inheritance is excluded, we see that
Nature has no need of the Lamarckian factor of modification for her
transformations and new adaptations.
If we wish to understand clearly that, in these changes, we have
to do not merely with the alteration of a single part, but of many
parts which all work together, we have only to think of the still
more striking physical modifications which have taken place in many
tropical ants, and which have led to a dimorphism of the workers.
In many species, certainly, the only difference is in size, so that
one can distinguish between large workers and small, and the former
are sometimes five times as big as the latter. But even in the South
European _Pheidole megalocephala_, which is abundant in Italy, the
larger workers are also different in structure from the smaller, for
they have an enormous head with powerful jaws. They are usually known
as 'soldiers,' and are entrusted with the defence of the colony.
Emery directly observed in regard to _Colobopsis truncata_, an ant
which lives in the trunks of trees, that the soldiers, with their
enormous heads, occupied all the entrances to the nest, ready to seize
any intruder with their powerful jaws. In the Sauba ant (_Œcodoma
cephalotes_) Bates described three different types of worker, differing
in size, and although he was not able to determine with certainty what
the particular function of each was, there can be no doubt that they
have special offices, and that the differences in their structure are
adaptations to the differences in their functions. The same is true of
the Indian ant, _Pheidologeton diversus_, depicted in Fig. 106, whose
three forms of workers I owe to the kindness of Professor August Forel.
If the increase in the size of the head and jaws must bring with it an
increase in the thickness of the skeleton of these parts, as well as
a strengthening of the musculature of the head, it follows that the
strain on the body must be greater, just as in the case of the increase
in the weight of the stag's antlers, so that the skeleton of the
thorax must likewise have become thicker and heavier, the muscles and
nerves of the legs stronger, the articulations of the joints capable
of greater resistance; in short, a whole series of variations of other
parts must have taken place simultaneously, if the primary variation
was to be of use, and not to lead to the destruction of its possessor.
Here again we have a proof that the co-adaptation of many parts can
take place without any intervention of the Lamarckian principle, and
that there must be some other factor which brings this about.
[Illustration: FIG. 106. Three workers of the same species of Indian
Ant (_Pheidologeton diversus_), drawn from specimens supplied by
Prof. August Forel. _A_, the largest, _B_, the intermediate, _C_, the
smallest form.]
Where, then, shall we look for this other factor, if not in the
processes of selection, in the selecting of the most suitable
variations among all those which occur? We are confronted with the
alternative of either working out a sufficient explanation with this
factor, or of giving up the attempt at explanation altogether. Yet the
application of the principle of selection in relation to the neuters
of colony-forming insects is by no means simple, for, as the workers
are sterile, a modification of them through processes of breeding
cannot begin directly with themselves. The workers which exhibit the
most suitable variations cannot be selected for breeding, but only
their parents, the sexual animals, and these according to whether
they produce better workers or worse. This is how Darwin looked at
the matter, and his view receives support from one peculiarity in
the composition of these animal colonies, whose significance becomes
apparent in relation to this problem. It has long been known that
in a bee-hive there are from 10,000 to 20,000 workers, but only one
true female, the so-called queen, and the meaning of this remarkable
arrangement probably is, that the adaptation of the workers through
natural selection becomes much more easily possible, _since the whole
number are the children of a single pair_. It is not the individual
workers, but the whole colony, that is, the whole progeny of the
queen, which is selected, according to the greater or less degree of
effectiveness displayed by the workers. Strictly speaking, it is the
single queen that is selected in relation to her power of producing
superior or inferior workers. A colony whose queen was unsatisfactory
in this respect could not hold its own in the struggle for existence,
and only the best colonies and the best hives would survive, that
is, through their descendants. If the hive contained a hundred
queens instead of a single queen the process of selection would be
much more complex and less clear, and it is even quite conceivable
that the production of specially modified workers, adapted to their
functions, or of two or three different kinds of workers, would not
have been possible at all. For it would not have helped much if one
out of a hundred females had produced workers of better structure;
only a majority of such females could give the colony any advantage as
compared with other colonies.
It has not been definitely established whether, among ants, a single
female is in all cases the founder of the whole colony, but it is
certain that there are only a few females. In the tropical Termites we
know that the ovaries of the female attain to such a colossal size that
one female must certainly suffice for the necessities of the largest
colonies. Grassi has shown, indeed, that, as far as the South European
Termites are concerned, not only are there several females present,
but that even the workers frequently reproduce; but the Termites in
general are inhabitants of warm countries, and the few European species
probably hardly represent the original composition of these animal
colonies. But of the tropical species, which have as yet not been
sufficiently studied, we know at least the extraordinary dimensions
of the body, and the corresponding fertility of the queens, and we
conclude from this that only a few can be present in each termitary[18].
[18] Ingwe Sjostedt has recently established in Africa that it is
usually a single queen and a single king that found a termitary
(_Schwed. Akad. Abh._ Bd. 34, 1902).
Now that we have discussed all these facts it will not be out of place
to summarize the results, in as far as they have any relation to the
acceptance or rejection of the theory of the inheritance of acquired
characters.
No _direct_ proof of such transmission could be found; on the contrary,
it has been shown that all that has hitherto been advanced as such will
not stand the test of close examination; an inheritance of wounds and
mutilations does not exist, the transmission of traumatically induced
epilepsy is not only doubtful as regards its causes, but cannot even
be considered as the transmission of a particular morphological lesion.
We may regard as _indirect_ proofs such facts as can only be explained
on the assumption of this mode of inheritance, and in this connexion
our opponents have cited especially the correspondence between
modifications acquired through use in the individual lifetime,
and worked out through histonal selection, with the phyletic
transformations of the same parts. But it has been shown that a number
of parts which do not function actively at all, but only passively, and
thus cannot be caused to change through use, like the hard skeletal
parts of the Arthropods, vary phyletically in the same certain and
direct course as those which function actively, so that we have every
ground for assuming that there are other factors operating in the
transformation of the active as well as of the passive parts. Finally,
we discussed the last and strongest argument which has been put forward
in favour of the Lamarckian principle, that of co-adaptation, that is,
the simultaneous adaptation of many parts co-operating in a common
action, and we were able to controvert this altogether by showing
that exactly similar phenomena of co-adaptation occur in systems
of passively functioning parts, and further, that they occur also
among the workers of ants and bees, that is, in animals which do not
reproduce, and which, therefore, cannot transmit the acquired results
of exercise during their life.
We therefore reject--and are compelled to reject--the Lamarckian
principle, not only on the ground that it cannot be proved correct, but
also because the phenomena, to explain which it is used, occur also
under circumstances which absolutely exclude any possibility of the
co-operation of this principle.
_Supplementary note on the Transmissibility of Acquired Functional
Modifications._
I cannot conclude this section without some reference to the utterances
of some naturalists who have quite recently attempted to represent the
inheritance of functional modifications as a conceivable and even a
necessary assumption.
I may name first Ludwig Zehnder, a physicist who has wandered into the
domain of biology. In regard to the very facts which I have adduced
as evidence _against_ the existence of such inheritance, he has
endeavoured to show how we might conceive of them as having by this
very means arisen[19].
[19] Zehnder, _Die Entstehung des Lebens_, Freiburg-i.-Br., 1899.
He deals with the case of ants, that is to say, with the
differentiation of the sterile workers into several castes, in the
following interesting manner.
The task of the workers is to procure all the food necessary for all
the individuals of the colony in quantity and quality corresponding
to the demand; failing this the whole colony would perish. Now the
different persons of the colony need different food, according to their
constitution and their functions. Soldiers, for instance, are more
powerful than ordinary workers, since they are adapted for fighting,
and they therefore require a different kind of food from the weaker
workers who are adapted only for other duties. Since the soldiers have
evolved from the latter by selection, what we may, for the sake of
brevity, call the soldier-food in the common stores of the ants would
be drawn upon more lavishly than before, and would therefore disappear
more quickly, and, whenever this occurred, those workers which had
already brought in this kind of food would be impelled to bring more
and more to satisfy the demand for it. But in order to do this they
would require to exert themselves more, and would therefore require a
larger quantity of food--not of course soldier-food, but the particular
kind which their particular qualities demanded. Probably this second
importation of food was undertaken by a second kind of worker, for,
according to Zehnder, each worker does not carry all the kinds of food;
they are divided into legions, each of which has its particular task of
food-collecting to fulfil.
In the end the storehouse of the ant-colony must contain a provision
in which the different kinds of food are in exact proportion to the
necessities of the different kinds of persons in the colony. It must
alter in its composition again as soon as, in the course of time, one
or other kind of person acquires new characters, for these presuppose a
new kind of diet.