(a) EXPERIMENTS ON THE MECHANISM OF HELIOTROPIC REACTIONS IN ANIMALS.
Since the instinctive reactions of animals are as hereditary as
their morphological character, a discussion of experiments on the
physico-chemical character of the instinctive reactions of animals
should not be entirely omitted from this sketch. It is obvious that such
experiments must begin with the simplest type of instincts, if they are
expected to lead to any results; and it is also obvious that only such
animals must be selected for this purpose, the reactions of which are
not complicated by associative memory, or, as it may preferably be
termed, associative hysteresis.
The simplest type of instincts is represented by the purposeful motions
of animals to or from a source of energy, e.g. light; and it is with
some of these that we intend to deal here. When we expose winged aphides
(after they have flown away from the plant), or young caterpillars of
Porthesia chrysorrhoea (when they are aroused from their winter sleep)
or marine or freshwater copepods and many other animals, to diffused
daylight falling in from a window, we notice a tendency among these
animals to move towards the source of light. If the animals are
naturally sensitive, or if they are rendered sensitive through the
agencies which we shall mention later, and if the light is strong
enough, they move towards the source of light in as straight a line as
the imperfections and peculiarities of their locomotor apparatus will
permit. It is also obvious that we are here dealing with a forced
reaction in which the animals have no more choice in the direction
of their motion than have the iron filings in their arrangement in a
magnetic field. This can be proved very nicely in the case of starving
caterpillars of Porthesia. The writer put such caterpillars into a glass
tube the axis of which was at right angles to the plane of the window:
the caterpillars went to the window side of the tube and remained there,
even if leaves of their food-plant were put into the tube directly
behind them. Under such conditions the animals actually died from
starvation, the light preventing them from turning to the food, which
they eagerly ate when the light allowed them to do so. One cannot say
that these animals, which we call positively helioptropic, are attracted
by the light, since it can be shown that they go towards the source
of the light even if in so doing they move from places of a higher to
places of a lower degree of illumination.
The writer has advanced the following theory of these instinctive
reactions. Animals of the type of those mentioned are automatically
orientated by the light in such a way that symmetrical elements of their
retina (or skin) are struck by the rays of light at the same angle.
In this case the intensity of light is the same for both retinae or
symmetrical parts of the skin.
This automatic orientation is determined by two factors, first a
peculiar photo-sensitiveness of the retina (or skin), and second
a peculiar nervous connection between the retina and the muscular
apparatus. In symmetrically built heliotropic animals in which the
symmetrical muscles participate equally in locomotion, the symmetrical
muscles work with equal energy as long as the photo-chemical processes
in both eyes are identical. If, however, one eye is struck by stronger
light than the other, the symmetrical muscles will work unequally and
in positively heliotropic animals those muscles will work with greater
energy which bring the plane of symmetry back into the direction of the
rays of light and the head towards the source of light. As soon as both
eyes are struck by the rays of light at the same angle, there is no more
reason for the animal to deviate from this direction and it will move in
a straight line. All this holds good on the supposition that the animals
are exposed to only one source of light and are very sensitive to light.
Additional proof for the correctness of this theory was furnished
through the experiments of G.H. Parker and S.J. Holmes. The former
worked on a butterfly, Vanessa antiope, the latter on other arthropods.
All the animals were in a marked degree positively heliotropic. These
authors found that if one cornea is blackened in such an animal, it
moves continually in a circle when it is exposed to a source of light,
and in these motions the eye which is not covered with paint is directed
towards the centre of the circle. The animal behaves, therefore, as if
the darkened eye were in the shade.
(b) THE PRODUCTION OF POSITIVE HELIOTROPISM BY ACIDS AND OTHER MEANS AND
THE PERIODIC DEPTH-MIGRATIONS OF PELAGIC ANIMALS.
When we observe a dense mass of copepods collected from a freshwater
pond, we notice that some have a tendency to go to the light while
others go in the opposite direction and many, if not the majority,
are indifferent to light. It is an easy matter to make the negatively
heliotropic or the indifferent copepods almost instantly positively
heliotropic by adding a small but definite amount of carbon-dioxide
in the form of carbonated water to the water in which the animals are
contained. If the animals are contained in 50 cubic centimetres of water
it suffices to add from three to six cubic centimetres of carbonated
water to make all the copepods energetically positively heliotropic.
This heliotropism lasts about half an hour (probably until all the
carbon-dioxide has again diffused into the air.) Similar results may be
obtained with any other acid.
The same experiments may be made with another freshwater crustacean,
namely Daphnia, with this difference, however, that it is as a rule
necessary to lower the temperature of the water also. If the water
containing the Daphniae is cooled and at the same time carbon-dioxide
added, the animals which were before indifferent to light now become
most strikingly positively heliotropic. Marine copepods can be made
positively heliotropic by the lowering of the temperature alone, or by a
sudden increase in the concentration of the sea-water.
These data have a bearing upon the depth-migrations of pelagic animals,
as was pointed out years ago by Theo. T. Groom and the writer. It is
well known that many animals living near the surface of the ocean or
freshwater lakes, have a tendency to migrate upwards towards evening and
downwards in the morning and during the day. These periodic motions are
determined to a large extent, if not exclusively, by the heliotropism
of these animals. Since the consumption of carbon-dioxide by the green
plants ceases towards evening, the tension of this gas in the water must
rise and this must have the effect of inducing positive heliotropism or
increasing its intensity. At the same time the temperature of the
water near the surface is lowered and this also increases the positive
heliotropism in the organisms.
The faint light from the sky is sufficient to cause animals which are in
a high degree positively heliotropic to move vertically upwards towards
the light, as experiments with such pelagic animals, e.g. copepods, have
shown. When, in the morning, the absorption of carbon-dioxide by the
green algae begins again and the temperature of the water rises, the
animals lose their positive heliotropism, and slowly sink down or become
negatively heliotropic and migrate actively downwards.
These experiments have also a bearing upon the problem of the
inheritance of instincts. The character which is transmitted in this
case is not the tendency to migrate periodically upwards and downwards,
but the positive heliotropism. The tendency to migrate is the outcome of
the fact that periodically varying external conditions induce a periodic
change in the sense and intensity of the heliotropism of these animals.
It is of course immaterial for the result, whether the carbon-dioxide or
any other acid diffuse into the animal from the outside or whether they
are produced inside in the tissue cells of the animals. Davenport and
Cannon found that Daphniae, which at the beginning of the experiment,
react sluggishly to light react much more quickly after they have been
made to go to the light a few times. The writer is inclined to attribute
this result to the effect of acids, e.g. carbon-dioxide, produced in the
animals themselves in consequence of their motion. A similar effect of
the acids was shown by A.D. Waller in the case of the response of nerve
to stimuli.
The writer observed many years ago that winged male and female ants
are positively helioptropic and that their heliotropic sensitiveness
increases and reaches its maximum towards the period of nuptial flight.
Since the workers show no heliotropism it looks as if an internal
secretion from the sexual glands were the cause of their heliotropic
sensitiveness. V. Kellogg has observed that bees also become intensely
positively heliotropic at the period of their wedding flight, in fact so
much so that by letting light fall into the observation hive from above,
the bees are prevented from leaving the hive through the exit at the
lower end.
We notice also the reverse phenomenon, namely, that chemical changes
produced in the animal destroy its heliotropism. The caterpillars of
Porthesia chrysorrhoea are very strongly positively heliotropic when
they are first aroused from their winter sleep. This heliotropic
sensitiveness lasts only as long as they are not fed. If they are kept
permanently without food they remain permanently positively heliotropic
until they die from starvation. It is to be inferred that as soon as
these animals take up food, a substance or substances are formed
in their bodies which diminish or annihilate their heliotropic
sensitiveness.
The heliotropism of animals is identical with the heliotropism of
plants. The writer has shown that the experiments on the effect of acids
on the heliotropism of copepods can be repeated with the same result in
Volvox. It is therefore erroneous to try to explain these heliotropic
reactions of animals on the basis of peculiarities (e.g. vision) which
are not found in plants.
We may briefly discuss the question of the transmission through the sex
cells of such instincts as are based upon heliotropism. This problem
reduces itself simply to that of the method whereby the gametes transmit
heliotropism to the larvae or to the adult. The writer has expressed the
idea that all that is necessary for this transmission is the presence in
the eyes (or in the skin) of the animal of a photo-sensitive substance.
For the transmission of this the gametes need not contain anything more
than a catalyser or ferment for the synthesis of the photo-sensitive
substance in the body of the animal. What has been said in regard to
animal heliotropism might, if space permitted, be extended, mutatis
mutandis, to geotropism and stereotropism.
(c) THE TROPIC REACTIONS OF CERTAIN TISSUE-CELLS AND THE MORPHOGENETIC
EFFECTS OF THESE REACTIONS.
Since plant-cells show heliotropic reactions identical with those
of animals, it is not surprising that certain tissue-cells also show
reactions which belong to the class of tropisms. These reactions of
tissue-cells are of special interest by reason of their bearing upon the
inheritance of morphological characters. An example of this is found in
the tiger-like marking of the yolk-sac of the embryo of Fundulus and in
the marking of the young fish itself. The writer found that the former
is entirely, and the latter at least in part, due to the creeping of the
chromatophores upon the blood-vessels. The chromatophores are at first
scattered irregularly over the yolk-sac and show their characteristic
ramifications. There is at that time no definite relation between
blood-vessels and chromatophores. As soon as a ramification of a
chromatophore comes in contact with a blood-vessel the whole mass of the
chromatophore creeps gradually on the blood-vessel and forms a complete
sheath around the vessel, until finally all the chromatophores form a
sheath around the vessels and no more pigment cells are found in the
meshes between the vessels. Nobody who has not actually watched the
process of the creeping of the chromatophores upon the blood-vessels
would anticipate that the tiger-like colouration of the yolk-sac in the
later stages of the development was brought about in this way. Similar
facts can be observed in regard to the first marking of the embryo
itself. The writer is inclined to believe that we are here dealing with
a case of chemotropism, and that the oxygen of the blood may be the
cause of the spreading of the chromatophores around the blood-vessels.
Certain observations seem to indicate the possibility that in the adult
the chromatophores have, in some forms at least, a more rigid structure
and are prevented from acting in the way indicated. It seems to the
writer that such observations as those made on Fundulus might simplify
the problem of the hereditary transmission of certain markings.
Driesch has found that a tropism underlies the arrangement of the
skeleton in the pluteus larvae of the sea-urchin. The position of this
skeleton is predetermined by the arrangement of the mesenchyme cells,
and Driesch has shown that these cells migrate actively to the place
of their destination, possibly led there under the influence of certain
chemical substances. When Driesch scattered these cells mechanically
before their migration, they nevertheless reached their destination.
In the developing eggs of insects the nuclei, together with some
cytoplasm, migrate to the periphery of the egg. Herbst pointed out that
this might be a case of chemotropism, caused by the oxygen surrounding
the egg. The writer has expressed the opinion that the formation of
the blastula may be caused generally by a tropic reaction of the
blastomeres, the latter being forced by an outside influence to creep to
the surface of the egg.
These examples may suffice to indicate that the arrangement of definite
groups of cells and the morphological effects resulting therefrom may
be determined by forces lying outside the cells. Since these forces are
ubiquitous and constant it appears as if we were dealing exclusively
with the influence of a gamete; while in reality all that it is
necessary for the gamete to transmit is a certain form of irritability.
(d) FACTORS WHICH DETERMINE PLACE AND TIME FOR THE DEPOSITION OF EGGS.
For the preservation of species the instinct of animals to lay their
eggs in places in which the young larvae find their food and can develop
is of paramount importance. A simple example of this instinct is the
fact that the common fly lays its eggs on putrid material which serves
as food for the young larvae. When a piece of meat and of fat of the
same animal are placed side by side, the fly will deposit its eggs upon
the meat on which the larvae can grow, and not upon the fat, on which
they would starve. Here we are dealing with the effect of a volatile
nitrogenous substance which reflexly causes the peristaltic motions for
the laying of the egg in the female fly.
Kammerer has investigated the conditions for the laying of eggs in two
forms of salamanders, e.g. Salamandra atra and S. maculosa. In both
forms the eggs are fertilised in the body and begin to develop in the
uterus. Since there is room only for a few larvae in the uterus, a large
number of eggs perish and this number is the greater the longer the
period of gestation. It thus happens that when the animals retain their
eggs a long time, very few young ones are born; and these are in a
rather advanced stage of development, owing to the long time which
elapsed since they were fertilised. When the animal lays its eggs
comparatively soon after copulation, many eggs (from 12 to 72) are
produced and the larvae are of course in an early stage of development.
In the early stage the larvae possess gills and can therefore live in
water, while in later stages they have no gills and breathe through
their lungs. Kammerer showed that both forms of Salamandra can be
induced to lay their eggs early or late, according to the physical
conditions surrounding them. If they are kept in water or in proximity
to water and in a moist atmosphere they have a tendency to lay their
eggs earlier and a comparatively high temperature enhances the tendency
to shorten the period of gestation. If the salamanders are kept in
comparative dryness they show a tendency to lay their eggs rather late
and a low temperature enhances this tendency.
Since Salamandra atra is found in rather dry alpine regions with a
relatively low temperature and Salamandra maculosa in lower regions with
plenty of water and a higher temperature, the fact that S. atra bears
young which are already developed and beyond the stage of aquatic life,
while S. maculosa bears young ones in an earlier stage, has been termed
adaptation. Kammerer's experiments, however, show that we are dealing
with the direct effects of definite outside forces. While we may speak
of adaptation when all or some of the variables which determine a
reaction are unknown, it is obviously in the interest of further
scientific progress to connect cause and effect directly whenever our
knowledge allows us to do so.