TROPISMS AND INSTINCTS AS ADAPTATIONS
Of the different kinds of adaptation none are more remarkable than those
connected with the immediate responses of organisms to external agents.
These responses are usually thought of as associated with the nervous
system; and while in the higher forms the nervous system plays an
important role in the reaction, yet in many cases it is little more than
the shortest path between the point stimulated and the muscles that
contract; and in the lower animals, where we find just as definite
responses, there may be no distinct nervous system, as in the protozoa,
for instance.
Many of the so-called instincts of animals have been shown in recent
years to be little more than direct responses to external agents. Many
of these instincts are for the good of the individual, and must be
looked upon as adaptations. For example: if a frog is placed in a jar of
water, and the temperature of the water lowered, the frog will remain at
the top until the water reaches 8 degrees C., when it will dive down to
the bottom of the jar; and, if the temperature is further lowered, it
will remain there until the water becomes warmer again, when it will
come to the surface again. It is clear that, under the ordinary
conditions of life of the frog, this reaction is useful to it, since it
leads the animal to go to the bottom of the pond on the approach of cold
weather, and thus to avoid being frozen at the surface.
Another illustration of an instinct that is a simple response to light
is shown by the earthworm. During the day the worm remains in its
burrow, but on dark nights it comes out of its hole, and lies stretched
out on the surface of the ground. It procures its food at this time, and
the union of the individuals takes place. In the early morning the worm
retires into its burrow.
This habit of the earthworm is the direct result of its reaction to
light. It crawls away from ordinary light as bright as that of diffuse
daylight, and, indeed, from light very much fainter than that of
daylight. If, however, the light be decreased to a certain point, the
worm will then turn and crawl toward the source of light. This lower
limit has been found by Adams to be about that of .001 candle-metre.
This corresponds to the amount of light of a dark night, and gives an
explanation of why the worm leaves its burrow only at night, and also
why it crawls back on the approach of dawn. It is also obvious that this
response is useful to the animal, for if it left the burrow during the
day, it would quickly fall a prey to birds.
The blow-fly lays its eggs on decaying meat, on which the larvæ feed.
The fly is drawn to the meat by its sense of smell, a simple and direct
response to a chemical compound given off by the meat. The maggot that
lives in the decaying meat is also attracted by the same odor, as Loeb
has shown, and will not leave the meat, or even a spot on a piece of
glass that has been smeared with the juice of the meat, so long as the
odor remains. Here again the life of the race depends on the proper
response to an external agent, and the case is all the more interesting,
since the response of the fly to the meat is of no immediate use to the
fly itself, but to the maggot that hatches from the egg of the fly.
The movement toward or from a stimulating agent is, in some cases,
brought about in the following way. Suppose an earthworm is lying in
complete darkness, and light be thrown upon it from one side. The worm
turns its head, as it thrusts it forward, to the side away from the
light; and as it again moves forward, it continues to bend its head away
from the light, until it is crawling directly away from the source. When
the light first strikes the worm, the two sides will be differently
illuminated. This causes a bending of the head, as it stretches forward,
toward the side of less illumination, and the bending is due to a
stronger contraction of some of the muscles on the less illuminated
side; at least the reaction appears to be due to a simple response of
this kind. When the body has been so far turned that the two sides are
equally illuminated, the muscles of the two sides will contract equally,
and the movement will be straight forward and away from the light. If
the reaction is as simple as this (which is in principle the explanation
advanced by Loeb), the result is a simple reflex act, and need not
involve any consciousness or intentional action on the part of the worm
to crawl away from the light. In fact, the same reaction takes place
when the brain is removed, not so quickly or definitely, it is true, but
this may be due to the removal of the anterior segments of the worm, in
which part the skin appears to be more sensitive to light than
elsewhere.
Another factor that plays an important rôle in the habits of the
earthworm is the response to contact,—the so-called stereotropism. If,
in crawling over a flat surface, the worm comes in contact with a
crevice, it will crawl along it, and refuse to leave until the end is
reached. The contact holds the worm as strongly as though it were
actually pulled into the crevice. It can be forced to leave a crevice
only by strong sunlight, and then it does not do so at once. If the worm
crawls into a small glass tube, it is also held there by its response to
contact, and the smaller the tube, the more difficult is it to make the
worm leave by throwing strong sunlight upon it.
Loeb has found that when winged aphids, the sexual forms, are collected
in a tube, and the tube is kept in a room, the aphids crawl toward the
light. This happens in ordinary diffuse light, as well as in lamplight.
It is stated that the animals orientate themselves towards the light
more quickly when it is strong than when it is weak. They turn their
bodies toward the light, and then move forward in the direction from
which the rays come. It can be shown by a simple experiment that the
aphids are turned by the _direction_ of the light, and not by its
intensity. If they are placed in a tube, and the tube laid obliquely
before a window in such a way that the direct sunlight falls only on the
inner end of the tube, the aphids will, if started at the inner end of
the tube, first crawl toward the outer surface of the tube, and then
wander along this wall, passing out of the region of sunlight into the
end of the tube nearest the window, where they come to rest at the end.
They have moved constantly towards the direction from which the rays
come, passing, as it were, from ray to ray, but each time toward a ray
nearer the source of the light.
If the tube be turned toward the window, and the window end be covered
with blue glass, the aphids crawl into this end of the tube, as they
would have done had the tube been uncovered. If, on the other hand, the
end of the tube be covered with red glass, they do not crawl into the
part of the tube that is covered, unless they are very sensitive to
light. Even in the latter case they may remain scattered in the red
part, and do not all accumulate at the end, as they do when blue glass
is used. In other words, while they respond to blue as they do to
ordinary light, they behave toward red as they do towards a very faint
light.
In diffuse daylight the aphids, as has been said, crawl toward the
light, but if they come suddenly into the sunlight they begin to fly.
Thus they remain on the food-plant until the sun strikes it, and then
they fly away.
The aphid also shows another response; it is negatively geotropic,
_i.e._ it tends to crawl upward against gravity. If placed on an
inclined, or on a vertical, surface, it will crawl upward. Such an
experiment is best made in the dark, since in the light the aphid also
responds to the light. If put on a window it crawls upward never
downward.
Aphids are also sensitive to heat. If they are placed in a darkened tube
and put near a stove, they crawl away from the warmer end; but if they
are acted upon by the light at the same time, they will be more strongly
attracted by the light than repulsed by the heat. We thus see that there
are at least three external agents that determine the movements of this
animal, and its ordinary behavior is determined by a combination of
these, or by that one that acts so strongly as to overpower the others.
The swarming of the male and female ants is also largely directed by the
influence of light. Loeb observed that when the direct sunlight fell
full upon a nest in a wall the sexual forms emerged, and then flew away.
Other nests in the ground were affected earlier in the day, because the
sun reached them first. These ants, when tested, were found to respond
to light in the same way as do the aphids. The wingless forms, or worker
ants, do not show this response, and the winged forms soon lose their
strong response to light after they have left the nest. Thus we see that
the heliotropism is here connected with a certain stage in the
development of the individual; and this is useful to the species, as it
leads the winged queens and males to leave the nest, and form new
colonies. Even the loss of response that takes place later may be looked
upon as beneficial to the species, since the queens do not leave the
nest after they have once established it.
It is familiar to every one that many of the night-flying insects are
attracted to a lamplight, and since those that fly most rapidly may be
actually carried into the flame before they can turn aside, it may seem
that such a response is worse than useless to them. The result must be
considered, however, in connection with other conditions of their life.
The following experiments carried out by Loeb on moths show some of the
responses of these insects to light.
Night-flying moths were placed in a box and exposed in a room to
ordinary light. As twilight approached the moths became active and began
to fly always toward the window side of the box. They were positively
heliotropic to light of this intensity. If let out of the case, they
flew toward the window, where they remained even during the whole of the
next day, fully exposed to light. If the moth is disturbed in the
daytime, so that it flies, it goes always toward the light, and never
away from it. These facts show that the moth is always positively
heliotropic, and also that the flight toward the lamp is a natural
response, misapplied in this case. That the moths do not fly by day is
due to another factor, namely, the alternation in the degree of their
sensitiveness at different times. But this condition alone does not seem
to account fully for all the facts.
If the moths are given the alternative of flying toward the evening
light, or toward the lamp, they always go toward the brighter light.
Thus if, when they swarm at dusk, they are set free in the middle of the
room, at the back of which a lamp is burning, the moths fly toward the
window. If, however, they are set free within a metre of the lamp, they
fly toward it.
The explanation that Loeb offers of the habit of these moths to fly only
in the evening is, that, although they are at all times positively
heliotropic, they respond to light only in the evening. In other words,
it is assumed that there is a periodic change in their sensitiveness to
light, which corresponds with the change from day to night. Loeb says
that, just as certain flowers open only at night, and others only during
the day, so do moths become more responsive in the evening, and
butterflies during the day. Both moths and butterflies are positively
heliotropic, and the sensitiveness of moths to light may be even greater
in the evening than is that of butterflies, for the light of the evening
to which the moth reacts is less than the minimal to which the butterfly
responds.
Moths appear to pass into a sort of sleep during the day, while
butterflies are quiescent only at night. The periodicity of the sleeping
time continues, at least for several days, when the insects are kept in
the dark. For instance, moths kept in the dark become restless as the
evening approaches, as Réaumur observed long ago. It has been found in
plants that this sort of periodicity may continue for several days, but
gradually disappears if the plants are kept in the dark. By using
artificial light, and exposing the plants to it during the night, and
putting them in the dark during the day, a new periodicity, alternating
with the former one, may be induced; and this will continue for some
days if the plants are then kept continually in the dark.
Loeb tried the experiment of exposing the quiescent moths suddenly to a
lower intensity of light, in order to see if they would respond equally
well at any time of day. It was found that if the change was made in the
forenoon, between six o’clock and noon, it was not possible to awaken
the moths by a sudden decrease in the intensity of the light. But it was
possible to do so in the afternoon, long before the appearance of dusk.
It appears, therefore, that in this species, _Sphinx euphorbiæ_, it is
possible to influence the period of awakening by decreasing the
intensity of light, but this can be done only near the natural period of
awakening. It seems to me that this awaking of a positively heliotropic
animal by decreasing the light needs to be further investigated.
The day butterflies are also positively heliotropic. Butterflies of the
species _Papilio machaon_, that have been raised from the pupa, remain
quietly on the window in the diffuse daylight of a bright day. They can
be carried around on the finger without leaving it, but the moment they
come into the direct rays of the sun they fly away.
Butterflies that have just emerged from their pupa case exhibit a marked
negative geotropic reaction, and this appears to be connected with the
necessity of unfolding their wings at this time. Loeb says that the same
cause that determines the direction of the falling stone and the paths
of the planets, namely, gravity, also directs the actions of the
butterfly that has just left its pupa case. The geotropic response is
especially strong at first. The animal wanders around until it reaches a
vertical wall, which it immediately ascends, straight upward, and
remains hanging at the top until its wings have unfolded. A similar
response occurs in the final stage of the larva of the May-fly, which
leaves the water and crawls up a blade of grass, or other vertical
support, and there, bursting the pupa skin, it dries its wings and flies
away. That this is a reaction to gravity and not to light is shown by
Loeb’s observation, that their empty skins are sometimes observed under
a bridge where the light does not come from above. “This observation on
the larva of the May-fly contradicts the assumption that the ‘purpose’
of the geotropic response of the butterfly is that it may the better
unfold its new wings, for in the ephemerid larva the negative geotropism
appears at a time when no wings are present.” On the other hand, it
should not be overlooked that the reaction is important for the May-fly
larva in other ways, because it leads the larva to leave the water at
the right period, and come out into the air, where the flying insect can
more safely emerge.
It is not without interest to find that caterpillars exhibit some of the
same reaction shown by butterflies. Loeb has made numerous experiments
with the caterpillars of _Porthesia chrysorrhœa_. The caterpillars of
this moth collect together in the autumn and spin a web or nest in which
they pass the winter. If they are taken from the nest and brought into a
warm room, they will orientate themselves to the light, and also crawl
toward it. If placed in a tube, they crawl to the upper side of the
glass and then along this side toward the light. If a covering is placed
over the end of the tube that is turned toward the window, the
caterpillars will crawl only as far as the edge of the cloth. They also
react negatively to gravity. If kept in a dark room, they will crawl
upward to the top of the receptacle in which they are enclosed. If
subjected to the influences of both light and gravity, they respond more
strongly to the light. The caterpillars also show a contact reaction.
They tend to collect on convex sides or on corners and angles of solid
bodies. They may even pile up one on top of the other in response to
this reaction; the convex side of a quiescent animal acting on another
animal crawling over it as any convex surface would do and holding the
animal fast.
These three kinds of reactions determine the instincts of these
caterpillars. In the spring, when they become warm, they leave the nest.
Positive heliotropism and negative geotropism compel them to crawl
upward to the tops of the branches of the trees, and there the contact
reaction with the small buds holds them fast in this place. That they
are not attracted to the end of the branches by the food that they find
there is shown by placing buds in the bottom of the tubes in which the
caterpillars are contained. The caterpillars remain at the top of the
tube, although food is within easy reach. If, however, they are placed
directly on the buds, the contact reaction will hold them there, and
they will not crawl farther upward. Curiously enough, as soon as the
caterpillars have fed and the time for shedding approaches, the
responsiveness to light and to gravity decreases, and at the time of
shedding they do not respond at all to these agents. These same
caterpillars react also to warmth above a certain point. In a dark tube
placed near a stove, the caterpillars collect at the end farthest away
from the source of the heat. They react to light best at a temperature
between 20 and 30 degrees C., and above this temperature point they
become restless and wander about.
The very close connection between the reactions of this caterpillar and
its mode of life is perfectly obvious. The entire series of changes
seems to have for its “purpose” the survival of the individual by
bringing it to the place where it will find its food. It may seem
natural to conclude that these responses have been acquired for this
very purpose, but let us not too quickly jump at this obvious conclusion
until the whole subject has been more fully examined.
The upward and downward movements of some pelagic animals have been
shown to depend on certain tropic responses. Every student of marine
zoology is familiar with the fact that many animals come to the surface
at night, and go down at the approach of daylight. It has been shown
that this migration is due largely to a response to light. Light can
penetrate to only about four hundred metres in sea-water, and there is
complete darkness below this level. It has been shown that the swimming
larvæ of one of the barnacles is positively heliotropic in a weak light,
but negatively heliotropic in a stronger light. Animals having responses
like these will come to the surface as the light fades away in the
evening and remain there until the light becomes too bright in the
following morning. They will then become negatively heliotropic and
begin to go down. When they reach a level where the intensity of the
light is such that they become positively heliotropic, they will turn
and start upward again. Thus during the day they will keep below the
surface, remaining in the region where they change from positive to
negative, and _vice versa_.
It would not be difficult to imagine that this upward and downward
migration of pelagic animals is useful to them, but, on the other hand,
it may be equally well imagined that the response may be injurious to
them. Thus it might be supposed that certain forms could procure their
food by coming to the surface at night, and avoid their enemies by going
down during the day. But it is difficult to see why organisms that serve
as prey should not have acquired exactly the opposite tropisms in order
to escape.
Some of these marine forms are also geotropic. Loeb has determined that
“the same circumstances that make the animals negatively heliotropic
also make them positively geotropic, and _vice versa_.” It was found,
for instance, that the larva of the marine worm Polygordius is
negatively geotropic at a low temperature, while at a higher temperature
it is positively geotropic. This response would drive the animals upward
when the water becomes too cold, and back again if the surface water
becomes too warm; but whether the response is so adjusted that the
animals keep, as far as possible, in water of that temperature that is
best for their development, we do not know. We can easily imagine that
within wide limits this is the case.
The change from positive to negative can also be brought about in other
ways. One of the most striking cases of this sort is that described by
Towle in one of the small crustaceans, _Cypridopsis vidua_. It was found
that after an animal had been picked up in a pipette its response was
always positive; that is, it swam toward the light, no matter what its
previous condition had been. The disturbance caused by picking the
animal up induced always a positive response towards light. If the light
were moved, the Cypridopsis followed the light. In this way it could be
kept positive for some time, but if it came to rest, or if it came into
contact with the sides or end of the trough, it became, after a short
time, negatively heliotropic, and remained negative as long as it could
be kept in motion, without being disturbed, or coming into contact with
a solid object. If when positive it were allowed to reach the glass at
the end of the trough, it would swim about there, knocking against the
glass, and then soon turn and swim away from the light. If the light
were shifted while the negative animal was in the middle of the trough,
it would turn and swim directly away, as before, from the source of
light. It could be kept in this negative state as long as it did not
come into contact with the ends.
It appears that the positive condition in Cypridopsis is of short
duration, and ceases after a while either as a response to contact or
without any observable external factor causing the change.
This crustacean lives at the bottom of pools, amongst water-plants, and
here also, no doubt, the same change from one to the other reaction
takes place. What possible advantage it may be to the animal to be kept
continually changing in this way is not at all obvious, nor, in fact,
are we obliged to assume that this reaction may be of any special use to
it. Indeed, it is far from obvious how the change that causes the animal
to swim toward the light when it is disturbed could be of the least
advantage to it.
In another crustacean, one of the marine copepods, _Labidocera æstiva_,
it has been shown by Parker that the male and female react in a somewhat
different way both to light and to gravity. The females are strongly
negatively geotropic, and this sends them up to the top of the water.
The males are very slightly negatively geotropic. The females are
strongly positively heliotropic toward light of low intensity; the males
show the same response to a less degree. To strong light the females are
negative and the males are indifferent. On the other hand, the males are
attracted to the females, probably in response to some chemical
substance diffusing from the females, since the males show the same
reaction when the females are enclosed in an opaque tube through whose
ends a diffusion of substances may take place. This crustacean frequents
the surface of the ocean from sunset to sunrise. During the day it
retires to deeper water. Its migrations can be explained as follows: The
females come to the surface at night, because they are positively
heliotropic to weak light, and also because they are negatively
geotropic. They go down during the day, because they react to bright
light more strongly than to gravity. The males follow the females,
largely because they react positively chemotactically toward the
females.
Some other animals respond in a somewhat different way to light, as
shown by the fresh-water planarians. These animals remain during the day
under stones, where the amount of light is relatively less than outside.
If they are placed in a dish in the light in front of a window, they
crawl away from the light, but when they reach the back of the dish they
do not come to rest, but continue to crawl around the sides of the dish
even toward the light. The light makes the worms restless, and while
they show a negative response as long as they are perfectly free to move
away from the light, they will not come to rest when they come to the
back of the dish if they are there still in the light, because the
irritating action of the light on them is stronger than its directive
action. If, however, in crawling about they come accidentally into a
place less bright than that in which they have been, they stop, and will
not leave this somewhat darker spot for a brighter one, although they
might leave the newly found spot for one still less bright.
At night the planarians come out and wander around, which increases
their chance of finding food, although it would not be strictly correct
to say that they come out in search of food. If, however, food is placed
near them, a piece of a worm, for example, they will turn toward it,
being directed apparently by a sense of smell, or rather of taste.
The heliotropic responses of the planarians appear to be of use to them,
causing them to hide away in the daytime, and to come out only after
dark, when their motions will not discover them to possible enemies. But
some of the planarians are protected in other ways, so that they will
not be eaten by fish, probably owing to a bad taste; so that it is not
so apparent that they are in real need of the protection that their
heliotropic response brings to them. Their turning towards their food
is, however, beyond question of great advantage to them, for in this way
they can find food that they cannot detect in any other way.
The unicellular plants were amongst the first organisms whose tropic
responses were studied, and the classical work of Strasburger gave the
impetus to much of the later work. In recent years the unicellular
animals, the protozoans, have been carefully studied, more especially by
Jennings. His results show that the reactions in these animals are
different in some important respects from those met with in higher
forms. For instance, most of the free-swimming infusoria are
unsymmetrical, as are also many of the flagellate forms, and as they
move forward they rotate freely on a longitudinal axis. It is therefore
impossible that they could orientate themselves as do the higher animals
that have been described above, and we should not expect these Protozoa
to react in the same way. In fact, Jennings shows that they exhibit a
different mode of response. Paramœcium offers a typical case. As it
moves forward it rotates toward the aboral side of the body. As a result
of the asymmetry of the body, the path followed, as it revolves on its
own axis, is that of a spiral. Did the animal not rotate, as it swims
forward, its asymmetrical form would cause it to move in a circle, but
its rotation causes, as has been said, the course to be that of a
spiral, and the general direction of movement is forward.[31] The
rotation of a paramœcium on its axis is in turn caused by the oblique
stroke of the cilia that cover the surface of the body. Their action
when reversed causes the animal to rotate backward.
Footnote 31:
The same result is attained by a bullet that is caused by the rifling
to rotate as it moves forward.
If a drop of weak acid be put into the water in which the paramœcia are
swimming,—for instance, in the water between a cover-slip and a
slide,—it will be found, after a time, that many individuals have
collected in the drop. It was at first supposed that the paramœcia are
attracted by the diffusion of the acid in the water, and turn toward the
source of the chemical stimulus; but Jennings has shown that this is not
the way in which the aggregation is brought about. If the individuals
are watched, it will be found that they swim forward in a spiral path
without regard to the position of the drop of acid. If one happens, by
chance, to run into the drop, there is no reaction as it enters, but
when it reaches the other side of the drop, and comes into contact with
the water on this side, it suddenly reacts. It stops, backs into the
middle of the drop, rotates somewhat toward the aboral side (_i.e._ away
from the vestibule), and then starts forward again, only to repeat the
action on coming into contact with the edge of the drop again. The
paramœcium has been caught in a veritable trap. All paramœcia that
chance to swim into the drop will also be caught, until finally a large
number will accumulate in the region. The result shows, that, in passing
from ordinary water into a weak acid, no reaction takes place; but
having once entered the acid, the animal reacts on coming into contact
with the water again.
On the other hand, there are some substances to which the paramœcium may
be said to be negatively chemotropic. If a drop of a weak alkaline
solution be put into water in which paramœcium is swimming, an
individual that happens to run against it reacts at once. It stops
instantly, backs off, revolving in the opposite direction, turns
somewhat to one side, and swims forward again. The chances are that it
will again hit the drop, in which case it repeats the same reaction,
turning again to one side. If it continues to react in this way, it
will, in the course of time, turn so far that when it swims forward it
will miss the edge of the drop, and then continue on its way. If an
individual were put into an alkaline drop, it would leave it, because it
would not react when it passed from inside the drop into the surrounding
water.
Unicellular animals react to other things besides differences in the
chemical composition of different parts of a solution. In many cases
they react to light, swimming toward or away from it according to
whether they are positively or negatively heliotropic. If they are
positively heliotropic, and while swimming run into a shadow, they react
as they would on coming into contact with a drop of acid. Since they
rotate as they swim forward, we cannot explain their orientation as in
the case of other animals that hold a fixed vertical position. If we
assume that the two ends of the body are differently affected by the
light, for which there is some evidence, we can perhaps in this way
account for their turning toward, or away from, the source of light.
Changes in the osmotic pressure of the different parts of the fluid,
mechanical stimulation produced by jarring, extremes of heat and of
cold, all cause this same characteristic reaction in Paramœcium; and
this accounts for their behavior toward these agents that are so
different in other respects.
Paramœcia, as well as other protozoans, show a contact response. They
fix themselves to certain kinds of solid bodies. If, for example, a
small bit of bacterial slime is put into the water, the paramœcia
collect around it in crowds, and eat the bacteria; but they will collect
in the same way around almost any solid. On coming in contact with
bodies having a certain physical texture, the cilia covering the
paramœcium stop moving, only those in the oral groove continuing to
strike backward. The animal comes to rest, pressed against the solid
body. If one or more paramœcia remain in the same place, they set free
carbon dioxide, as a result of their respiratory processes. There is
formed around them a region containing more of this acid than does the
surrounding water. If other moving paramœcia swim, by chance, into this
region, they are caught, and as a result an accumulation of individuals
will take place. The more that collect the larger will the area become,
and thus large numbers may be ultimately entrapped in a region where
there is formed a substance that, from analogy with other animals, we
should expect to be injurious.
The question as to how far these responses of the unicellular forms are
of advantage to them is difficult to decide, for while, as in the above
case, the response appears to be injurious rather than useful, yet under
other conditions the same response may be eminently advantageous. In
other cases, as when the paramœcia back away, and then swim forward
again, only to repeat the process, the act appears to be such a stupid
way of avoiding an obstacle that the reaction hardly appears to us in
the light of a very perfect adaptation. If we saw a higher animal trying
to get around a wall by butting its head into it until the end was
finally reached, we should probably not look upon that animal as well
adapted for avoiding obstacles.
Bacteria, which are generally looked upon as unicellular plants, appear,
despite the earlier statements to the contrary, to react in much the
same way as do the protozoans, according to the recent work of Rothert,
and of Jennings and Crosby. The bacteria do not seem to turn toward or
away from chemical substances, but they collect in regions containing
certain substances in much the same way as do the protozoans. The
collecting of bacteria in regions where oxygen is present has been known
for some time, but it appears from more recent results that they are not
attracted toward the oxygen, but by accidentally swimming into a region
containing more oxygen they are held there in the same way as is
paramœcium in a drop of acid. On the other hand bacteria do not enter a
drop of salt solution, or of acids, or of alkalies. They react
negatively to all such substances. Some kinds of bacteria have a
flagellum at each end, and swim indifferently in either direction. If
they meet with something that stimulates them, as they move forward,
they swim away in the opposite direction, and continue to move in the
new direction until something causes again a reversal of their movement.
In this respect their mode of reaction seems of greater advantage than
that followed by paramœcium.
Another instinct, that appears to be due to a tropic response, is the
definite time of day at which some marine animals deposit their eggs.
The primitive fish, Amphioxus, sets free its eggs and sperm only in the
late afternoon. A jellyfish, Gonionema, also lays its eggs as the light
begins to grow less in the late afternoon, and in this case it has been
found that the process can be hastened if the animals are placed in the
dark some hours before their regular time of laying. There is no
evidence that this habit is of any advantage to the animal. We may
imagine, if we like, that the early stages may meet with less risk at
night, but this is not probable, for it is at this time that countless
marine organisms come to the surface, and it would seem that the chance
of the eggs being destroyed would then be much greater. It is more
probable that the response is of no immediate advantage to the animals
that exhibit it, although in particular cases it may happen to be so.
This response recalls the diurnal opening and closing of certain
flowers. The flowers of the night-blooming cereus open only in the dusk
of evening, and then emit their strong fragrance. Other flowers open
only in the daytime, and some only in bright sunlight. It is sometimes
pointed out that it is of advantage to some of these flowers to open at
a certain time, since the particular insects that are best suited to
fertilize them may then be abroad. This may often be the case, but we
cannot but suspect that in other cases it may be a matter of little
importance. In special instances it may be that the time of opening of
the flowers is of importance to the species; but even if this is so,
there is no need to assume that the response has been gradually acquired
for this particular purpose. If it were characteristic of a new form to
open at a particular time, and there were insects in search of food at
this time that would be likely to fertilize the plant, then the plant
would be capable of existing; but this is quite different from supposing
that the plant developed this particular response, because this was the
most advantageous time of day for the fertilization of its flowers.
We can apply this same point of view, I believe, to many of the
remarkable series of tropisms shown by plants, whose whole existence in
some cases is closely connected with definite reactions to their
environment. Let us examine some of these cases.
When a seed germinates, the young stem is negatively geotropic, and, in
consequence, as it elongates it turns upward towards the light that is
necessary for its later growth. The root, on the contrary, is positively
geotropic, and, in consequence, it is carried downward in the ground.
Both responses are in this case of the highest importance to the
seedling, for in this way its principal organs are carried into that
environment to which they are especially adapted. It matters very little
how the seed lies in the ground, since the stem when it emerges will
grow upward and the root downward. The young stem, when it emerges from
the soil, will turn toward the light if the illumination comes from one
side, and this also may often be of advantage to the plant, since it
turns toward the source from which it gets its energy. The leaves also
turn their broad surfaces toward the light, and as a result they are
able to make use of a greater amount of the energy of the sunlight. The
turning is due to one side of the stem growing more slowly than the
opposite side, and it is true, in general, that plants grow faster at
night than in the daylight. Very bright light will in some cases
actually stop all growth for a time. Thus we see that this bending of
the stem toward the light and the turning of the leaves to face the
light are only parts of the general relation of the whole plant toward
the light.
Negative heliotropism is much less frequent in plants. It has been
observed in aërial roots, in many roots that are ordinarily buried in
the ground, in anchoring tendrils that serve as holdfasts, and even in
the stems of certain climbers. In all of these cases, and more
especially in the case of the climbers, the reaction is obviously of
advantage to the plant; and it is significant to find, in plants that
climb by tendrils carrying adhering disks, that there is a reversal of
the ordinary heliotropism shown by homologous organs in other plants.
There is an obvious adaptation in the behavior of the tendril, since its
growth away from the more illuminated side is just the sort of reaction
that is likely to bring it into contact with a solid body.
In this connection it is important to observe that these reactions to
light are perfectly definite, being either positive or negative under
given conditions, and therefore there is at present nothing to indicate
that there has been a gradual transformation from positive to negative,
or _vice versa_. It seems to me much more probable that when the
structural change took place, that converted the plant into a climber,
there appeared a new heliotropic response associated with the other
change. In other words, both appeared together in the new organ, and
neither was gradually acquired by picking out fluctuating variations.
The leaves of plants also show a sort of transverse heliotropic
response. It has been found, for example, that the leaves of Malva will
turn completely over if illuminated by a mirror from below. A curious
case of change of heliotropism is found in the flower stalks of Linaria.
They are at first positively heliotropic, but after the flower has been
fertilized the stalk becomes negatively heliotropic. As the stalks
continue to grow longer, they push the fruits into the crevices of the
rocks on which the plants grow, and in this way insure the lodgement of
the seeds. Here we have an excellent example showing that the negative
heliotropism of the flower stalk could scarcely have been acquired by
slight changes in the final direction, for only the complete change is
useful to the plant. Intermediate steps would have no special value.
As has been pointed out in the case of the seedling plant, the main stem
responds positively and the roots negatively to gravity. In addition to
this, the lateral position taken by the lateral roots and branches and
by underground stems are also, in part, due to a geotropic response. In
this case also the effect is produced by the increased growth on the
upper side when the response is positive, and on the lower when it is
negative. Leaves also assume a transverse position in response to the
action of gravity, or at least they make a definite angle with the
direction of its action.
The most striking case of geotropic response is seen in plants that
climb up the stems of other plants. The twining around the support is
the result of a geotropic response of the sides of the stem. The young
seedling plant stands at first erect. As its end grows it begins to
curve to one side in an oblique position, and this is due to an increase
in growth on one side of the apex of the shoot. As a result the stem
bends toward the other side. Not only does the end “sweep round in a
circle like the hands of a watch,” but it rotates on its long axis as it
revolves. As a result of this rotation “the part of the stem subjected
to the action of the lateral geotropism is constantly changing; and the
revolving movement once begun, must continue, as no position of
equilibrium can be attained.” This movement will carry the end around
any support, not too thick, that the stem touches.
Most climbers turn to the left, _i.e._ against the hands of a watch,
others are dextral, and a few climb either way.[32] Strasburger states
that whenever any external force, or substance, is important to the
vital activity of the plant or any of its organs, there will also be
found to be developed a corresponding irritability to their influence.
Roots in dry soil are diverted to more favorable positions by the
presence of greater quantities of moisture. This may, I venture to
suggest, be putting the cart before the horse. The plant may be only
able to exist whose responses are suited to certain external conditions,
and these determine the limits of distribution of the plant or the
places in which it is found.
Footnote 32:
These cases recall the spiral growth of the shell of the snail, but
the spiral in the latter is due to some other factor.
A number of plants climb in a different way, and show another sort of
tropism. Those that climb by means of tendrils twist their tendrils
about any support that they happen to come in contact with, and thus the
plant is able to lift its weak stem, step by step, into the air. The
twining of the tendrils is due to contact, which causes a cessation of
growth at the points of contact. The growth of the opposite side
continues, and thus the tendril bends about its support. In the grape
and in ampelopsis the tendril is a modified branch. The stalk of the
leaves in a few plants, as in Lophospermum, act as tendrils. Other
climbers are able to ascend vertical walls owing to the presence of
disks, whose secretions hold the tendril firmly against the support, as
in ampelopsis.
It is interesting to find in practically all these cases that, whatever
the stimulus may be, the results are reached in the same way, namely, by
one part growing faster than another. The fact of importance in this
connection is that the plant is so constructed that the response is
often beneficial to the organism.
Before leaving this subject there is one set of responses to be referred
to that is not the result of growth. Certain movements are brought about
by the change in the turgidity of certain organs. The small lateral
leaflets of _Desmodium gyrans_ make circling movements in one to three
minutes. No apparent benefit results from their action. The terminal
leaflets of _Trifolium pratense_ oscillate in periods of two to four
hours, but do so only in the dark; in the light the leaflets assume a
rigid position. There is nothing in the process to suggest that the
movement is useful to the plant, and yet it appears to be as definite as
are those cases in which the response is of vital importance. Had these
movements been of use, their origin would, no doubt, have been explained
because of their usefulness, and the conclusion would have been wrong.
The leaves of the Mimosa respond, when touched, and it cannot be
supposed that this is of any great advantage to the plant. The sleep
movements of many plants are also due to the effect of light. In some
cases the leaflets are brought together with their upper surfaces in
contact with one another; in other cases the lower surfaces are brought
together. Darwin supposed that these sleep movements served to protect
the leaves from a too rapid loss of heat through radiation, but it has
been pointed out that tropical plants exhibit the same responses. We
have here another admirable instance of the danger of concluding that
because we can imagine an advantage of a certain change, that the change
has, therefore, been acquired because of the advantage. In the Mimosa
not only do the leaflets close together, but the whole leaf drops down
if the stimulus is strong. Other plants also show in a less degree the
same movements, Robinia and Oxalis for instance, and certainly in these
latter the result does not appear to be of any advantage to the plants.
The preceding account of some of the tropisms in animals and plants will
serve to give an idea of how certain movements are direct responses to
the environment. Some of the reactions appear to be necessary for the
life of the individual, others seem to be of less importance, and a few
of no use at all. Yet the latter appear to be as definite and
well-marked as are the useful responses. I think the conviction will
impress itself on any one who examines critically the facts, that we are
not warranted in applying one explanation to those responses that are of
use, and another to those that are of little or of no value. Inasmuch as
the Darwinian theory fails to account for the origin of organs of little
or of no value, it is doubtful if it is needed to explain the origin of
the useful responses. If, on the other hand, we assume that the _origin_
of the responses has nothing to do with their value to the organism, we
meet with no difficulty in those cases in which the response is of
little or of no use to the organism. That great numbers of responses are
of benefit to the organism that exhibits them can be accounted for on
the grounds that those new species, that have appeared, that have useful
responses, are more likely, in the long run, to survive, than are those
that do not respond adaptively.
We may now examine some of the more complicated responses and instincts,
more especially those of the higher animals. Some of these are pure
tropisms, _i.e._ definite responses or reactions to an external exciting
agent; others may be, in part, the result of individual experience,
involving memory; others, combinations of the two; and still others may
depend on a more complex reaction in the central nervous system of the
animal. These cases can be best understood by means of a few
illustrations.
As an example of a simple action may be cited a well-known reflex after
cutting the nerve-cord of the frog, or after destroying the brain. If
the frog is held up, and its side tickled, the leg is drawn up to rub
the place touched. To accomplish this requires a beautifully adjusted
system of movements, yet the act seems to be a direct reflex, involving
only the spinal cord.
An example of a somewhat more complex reflex is the biting off of the
navel-string by the mother in rodents and other mammals; an act
eminently useful to the young animal, although of no importance to the
mother herself. The protection of the young by their parents from the
attacks of other animals appears to be a somewhat complex instinct, and
it is interesting to note that the protection is extended to the young
only so long as they are in need of it, and as soon as they are able to
shift for themselves the maternal protection is withdrawn.
The instinct of the young chick to seize in its beak any small moving
object is a simple and useful reflex action, but if the object should
happen to be a bee which stings the chick, another bee or similar insect
will not be seized. Here we see that a reflex has been changed, and
changed with amazing quickness. Moreover, the chick has learnt to
associate this experience with a particular sort of moving object. It is
this power to benefit by the result of a brief experience that is one of
the most advantageous properties of the organism.
Young chicks first show a drinking reflex if by chance their beaks are
wet by water. At once the head is lifted up, and the drop of water
passes down the throat. In this way the chick first learns the meaning
of water, and no doubt soon comes to associate it with its own condition
of thirst. The sight of water produces no effect on the inexperienced
chick, and it may even stand with its feet in the water without
drinking; but as soon as it touches, by chance, the water with its beak,
the reflex, or rather the set of reflexes is started.
A more complicated instinct is that shown by the spider in making its
web. In some cases the young are born from eggs laid in the preceding
summer, and can have had, therefore, no experience of what a web is
like; and yet, when they come to build this wonderfully complex
structure, they do so in a manner that is strictly characteristic of the
species.
The formation of the comb by bees, in which process, with a minimum of
wax, they secure a maximum number of small storehouses in which to keep
their honey and rear their young, is often cited as a remarkable case of
adaptation.
There has been some discussion as to whether birds build their nests in
imitation of the nest in which they were reared, or whether they do so
independently of any such experience. There can be no doubt, however,
that in some birds neither memory nor imitation can play any important
part in the result, and that they build their nests as instinctively as
spiders make webs.
These instincts of spiders, bees, and birds appear to be more complex
than the reflexes and tropisms that were first described. Whether they
are really so, or only combinations of simple responses, we do not yet
know. That they have come suddenly into existence as we now find them
does not seem probable, but this does not mean that they must have been
slowly acquired as the result of selection. The mutation theory also
assumes that the steps of advance may have been small.
Our account may be concluded with the recital of some instincts, chosen
almost at random, that serve to show some other adaptations which are
the result of these inborn responses.
It is known that ants travel long distances from their nests, and yet
return with unerring accuracy. It has been shown that they are able to
do this through a marvellous sense of smell. The track left by the ant,
as it leaves the nest, serves as a trail in returning to the
starting-point. Moreover, it appears that the ant can pick out her own
trail, even when it has been crossed by that of other ants. This means
that she can distinguish the odor of her own trail from that of other
members of the colony. The sense-organs by means of which the odor is
detected lie in the antennæ. This fact accounts for certain actions of
ants that have been described as showing that they have an affection for
each other. Two ants, meeting, pat each other with their antennæ. In
this way they are quickly able to distinguish members of their own nest
from those of other nests. If they are of the same nest, they separate
quietly; if of other nests, they may fight. If an ant from one nest is
put into another nest, it is instantly attacked and killed—an act that
appears to be injurious rather than useful, for the ant might become a
valuable member of the new colony. If, however, an ant is first immersed
in the blood of a member of the community into which she is to be
introduced, she will not be attacked, and may soon become a part of the
new community. By her baptism of blood she has no doubt acquired
temporarily the odor of the new nest, and by the time that this has worn
off she will have acquired this odor by association, and become thereby
a member of another colony.
Numerous stories have been related of cases in which an ant, having
found food, returns to the nest with as much of it as she can carry, and
when she comes out again brings with her a number of other ants. This
has been interpreted to mean that in some mysterious way the ant
communicates her discovery to her fellow-ants. A simpler explanation is
probably more correct. The odor of the food, or of the trail, serves as
a stimulus to other ants, that follow to the place where the first ant
goes for a new supply of the food. The fact that the first individual
returns to the supply of food seems to indicate that the ant has memory,
and this is obviously of advantage to her and to the whole colony.
The peculiar habits of some of the solitary wasps, of stinging the
caterpillar or other insect which they store up as food for their young,
is often quoted as a wonderful case of adaptive instinct. The poison
that is injected into the wound paralyzes the caterpillar, but as a rule
does not kill it, so that it remains motionless, but in a fresh state to
serve as food for the young that hatch from the egg of the wasp. A
careful study of this instinct by Mr. and Mrs. Peckham has shown
convincingly that the act is not carried out with the precision formerly
supposed. It had been claimed that the sting is thrust into the
caterpillar on the lower side, a ventral ganglion being pierced, the
poison acting with almost instantaneous effect. But it may be questioned
whether this is really necessary, and whether the same end might not be
gained, although not quite so instantaneously, if the caterpillar were
pierced in almost any other part of the body. Can we be seriously asked
to believe that this instinct has been perfected by the destruction of
those individuals (or of their descendants) that have not pierced the
caterpillar in exactly the middle of a segment of the anterior ventral
surface? It seems to me that the argument proves too much from the
selectionist’s point of view. If the wasp pierced the caterpillar in the
middle of its back, we should have passed over the act without comment;
but since the injection is usually made on the ventral side, and since
we know that the nervous system lies in this position, it has been
assumed that the act is carried out in this way, in order that the
poison may penetrate the nervous system more quickly. Yet a fuller
knowledge may show that there is really no necessity for such precision.
A curious response is the so-called death-feigning instinct shown by a
number of animals, especially by certain insects, but even by some
mammals and birds. Certain insects, if touched, draw in their legs, let
go their hold, and fall to the ground, if they happen to be on a plant.
It is not unusual to meet with the statement that this habit has been
acquired because it is useful to the insect, since it may often escape
in this way from an enemy. This does not appear on closer examination to
be always the case, and sometimes as much harm as good may result, or
what is more probable, neither much advantage, nor disadvantage, is the
outcome. This can, of course, only be determined in each particular case
from a knowledge of the whole life of a species and of the enemies that
are likely to injure it.
Hudson has recorded[33] a number of cases of this death-feigning
instinct in higher animals, and attributes it to violent emotion, or
fear, that produces a sort of swoon. He describes the gaucho boys’
method, in La Plata, of catching the silver-bill by throwing a stick or
a stone at it, and then rushing toward the bird, “when it sits perfectly
still, disabled by fear, and allows itself to be taken.” He also states
that one of the foxes (_Canis azaræ_) and one of the opossums
(_Didelphys azaræ_) “are strangely subject to the death-simulating
swoon.”
Footnote 33:
“The Naturalist in La Plata.”
Hudson remarks that it seems strange that animals so well prepared to
defend themselves should possess this “safeguard.” When caught or run
down by dogs, the fox fights savagely at first, but after a time its
efforts stop, it relaxes, and it drops to the ground. The animal appears
dead, and Hudson states that the dogs are “constantly taken in by it.”
He has seen the gauchos try the most barbarous tricks on a captive fox
in this condition, and, despite the mutilations to which it was
subjected, it did not wince. If, however, the observer draws a little
away from the animal, “a slight opening of the eye may be detected, and
finally, when left to himself, he does not recover and start up like an
animal that has been stunned, but cautiously raises his head at first
and only gets up when his foes are at a safe distance.” Hudson, coming
once suddenly upon a young fox, saw it swoon at his approach, and
although it was lashed with a whip it did not move.
The common partridge of the pampas of La Plata (_Hothura maculosa_)
shows this death-feigning instinct in a very marked degree. “When
captured, after a few violent struggles to escape, it drops its head,
gasps two or three times, and to all appearance dies.” But if it is
released it is off in an instant. The animal is excessively timid, and
if frightened, may actually die simply from terror. If they are chased,
and can find no thicket or burrow into which to escape, “they actually
drop down dead on the plain. Probably when they feign death in their
captor’s hand they are in reality very near to death.”
In this latter instance it must appear very improbable that we are
dealing with an instinct that has been built up by slow degrees on
account of the benefit accruing at each stage to the individual. In
fact, it appears that the instinct is in this case of really no use at
all to the animal, for there can scarcely be any question of an escape
by this action. Yet so far as we can judge it is the same instinct shown
by other animals, and it is not logical to account for its origin in one
case on the grounds of its usefulness, when we cannot apply the
explanation in the other cases. If this be admitted, we have another
illustration of the importance of keeping apart the origin of an
instinct or of a structure and the fact of its usefulness or
non-usefulness to the organism. Thus under certain conditions this
death-feigning instinct might really be of use to the animal, while
under other conditions and in other animals it may be of no advantage at
all, and in still other conditions it may be a positive injury to its
possessor. Perhaps we need not go outside of our own experience to find
a parallel case, for the state of fright into which imminent danger may
throw an individual may deprive him for the moment of the proper use of
those very mental qualities of which he stands in this crisis in
greatest need.
The peculiar behavior of cattle caused by the smell of blood is another
case of an instinct whose usefulness to its possessors is far from
apparent. It is known that cattle and horses and several wild animals
become violently excited by the smell of blood. Hudson gives a vivid
account of a scene witnessed by himself, the animals congregating, “and
moving around in a dense mass, bellowing continually.” Those animals
that forced their way into the centre of the mass where the blood was
“pawed the earth and dug it up with their horns, and trampled each other
down in their frantic excitement.”
This action leads us to a consideration of the behavior of animals
toward companions in distress. “Herbivorous animals at such times will
trample and gore the distressed one to death. In the case of wolves and
other savage-tempered carnivorous species the distressed fellow is
frequently torn to pieces and devoured on the spot.” If any one will be
bold enough to claim in this case that this habit has been acquired
because of advantage to the pack, _i.e._ if it be imagined that the pack
gains more by feeding on a weak member than by letting him take his
chances of recovery, it may be pointed out in reply that cattle also
destroy their weak or injured, but do not devour them, and the same
statement holds for birds, where the same instinct has often been
observed. Romanes has suggested that the instinct of destroying the weak
or injured members is of use because such members are a source of danger
to the rest of the herd; but Hudson points out that it is not so much
the weak and sickly members of the herd that are attacked in this way,
as those that are injured, and concludes, “the instinct is not only
useless, but actually detrimental.” He suggests that these “wild
abnormal movements of social animals” are a sort of aberration, so “that
in turning against a distressed fellow they oppose themselves to the law
of being.” Yet whether we gain anything by calling this action aberrant
or abnormal, the important fact remains that it is a definite response
under certain external conditions, and is shown by all the individuals
of the species.
The preceding illustrations of reactions that go to make up the
so-called instincts of animals may be separated into those that are
essential to the life of the individual or of the race, those that are
of some apparent use, although not absolutely essential, and a few of no
use at all, and fewer still that appear to be even injurious. If the
latter reactions take place only rarely, as appears often to be the
case, they are not sufficiently harmful to cause the destruction of the
race. The evidence points to the conclusion, I believe, that the origin
of these tropisms and instincts cannot be accounted for on the ground of
their benefit to the individual or to the race; and it does not seem
reasonable to make up one explanation for the origin of those that are
essential, and another for those that are of little use or even of no
use at all.
From what has been already said more than once, while discussing each
particular case, the simplest course appears to be in all instances to
look upon these instincts as having appeared independently of the use to
which they may be put, and not as having been built up by selection of
the individual variations that happen to give an organism some advantage
over its fellows in a life and death struggle. It appears reasonable to
deal with the origin of tropisms and instincts in general in the same
way as in dealing with structures; for, after all, the tropism is only
the outcome of some material or structural basis in the organism.
No attempt has been made here to interpret the more complex reactions of
the nervous system, for until we can get some insight into the meaning
of the simpler processes, we are on safer ground in dealing with these
first.
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