Darwin and Modern Science [185]
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,
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,