The Mechanistic Conception of Life

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 toward the center of the circle. The animal behaves, therefore, as if the darkened eye were in the shade.

h) The production of positive heliotropism hy acids and other means and the periodic depth migrations of pelagic animals. — When we observe a dense mass of copepods collected from a fresh-water 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 w^ater in which the animals are contained. If the animals are contained in 50 c.c. of water it suffices to add from 3 to 6 c.c. 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. IMarine 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 kno\Mi that many animals living near the surface of the ocean or fresh-water lakes, have a tendency to migrate upward toward evening and do'^Tiward 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 toward 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 upward toward the light, as experiments with such pelagic animals, e.g., copepods, have shoAMi. 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 doAMi or become negatively heliotropic and migrate actively do^^•nward.

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 upward and do\\Tiward, but the positive heliotropism. The tendency to migrate is the outcome of the fact that periodically varjdng 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 a nerve to stimuli.

The writer observed many years ago that winged male and female ants are positively heliotropic and that their heliotropic sensitiveness increases and reaches its maximum toward 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, the formation of a substance or substances in their bodies takes place, diminishing or annihilating their heliotropic sensitiveness.

The heliotropism of animals is identical ^\^th the heliotropism of plants. The writer has sho\\Ti 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 of a photosensitive substance in the eyes (or in the skin) of the animal. For the transmission of this the gametes need not contain anything more than a catalyzer or ferment for the synthesis of the photosensitive 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 (Fig. 36, p. 106). 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 (Fig. 37) and forms a complete sheath aromid 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 (Fig. 38). Nobody who has not actually watched the process of the creeping of the chromatophores upon the blood-vessels would anticipate that the tiger-like coloration of the yolk sac in the later stages of 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 showTi 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 peripherj^ 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 general^ 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 maj^ 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 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. AVhen 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 laj^ng of eggs in two forms of salamanders, e.g., Salatnandra atra and S. maculosa. In both forms the eggs are fertilized in the body and begin to develop in the uterus. Smce 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 fertilized. When the animal lays its eggs comparatively soon after copulation, man}- eggs (from twelve to

seventy-two) are produced and the larvae are of course in an earty 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 dr^mess 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 inaculosa 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.

The discovery of DeVries, that new species may arise by mutation and the wide if not universal applicability of Mendel's law to phenomena of heredity, as shown especially by Bateson and his pupils, must, for the time being, if not permanently, serve as a basis for theories of evolution. These discoveries place before the experimental biologist the definite task of producing mutations by physico-chemical means. It is true that certain authors claim to have succeeded in this, but the writer wishes to apologize to these authors for his inability to convince himself of the validity of their claims at the present moment. He thinks that only continued breeding of these apparent mutants through several generations can afford convincing evidence that we are here dealing with mutants rather than with merely pathological variations.^

What was said in regard to the production of new species by physico-chemical means may be repeated with still more justification in regard to the second problem of transformation, namely, the making of living from inanimate matter. The purely morphological imitations of bacteria or cells which physicists have now and then proclaimed as artificially produced living beings, or the plays on words by which, e.g., the regeneration of broken crystals and the regeneration of lost limbs by a crustacean were declared identical will not appeal to the biologist. We know that growth and development in animals and plants are determined by definite although complicated series of catenary chemical reactions, which result in the s^mthesis of a definite compound or group of compounds, namely, nucleins.

The nucleins have the peculiarity of acting as ferments or enzymes for their own synthesis. Thus a given type of nucleus will continue to synthesize other nuclein of its o^vn kind. This determines the continuity of a species; since each species has, probably, its own specific nuclein or nuclear material. But it also shows us that whoever claims to have succeeded in making living matter from inanimate will have to prove that he has succeeded in producing nuclear material which acts as a ferment for its own synthesis and thus reproduces itself. Nobody has thus far succeeded in this, although nothing warrants us in taking it for granted that this task is beyond the power of science.

1 Since this was written the beautiful experiments of Kammerer as well as those of Tower seem to have furnished proof that external conditions can cause hereditary changes in animals. of, 179 ff. Action of potassium cyanide, 156 flf. Activation of the egg, 6 fif. Agalma, 208. Amphitrite, 144. Animal instincts, experimental control Artificial causation of positive heliotropism, 43, 220. Artificial parthenogenesis, 7, 116 flf., 100 flf. Artificial production of living matter, 5. Ascidians, 68.

Changes in color of butterflies produced through influence of temperature, 211. Color, changes in, of butterflies, through influence of temperatiu*e, 211; color adaptation, 80 flf., 213 fif.; color blindness, 16. Cooperative action of salts causing impermeability of the egg membrane, 176 flf. Cortical layer, of unfertilized egg, 10, 189; mechanical destruction of, chemical processes, 209 flf. Difference of salt permeability of Ear, otoliths of, and orientation to center of gravity of the earth, 57.

Effect of retarded oxidations on poisonous salt action, 190 ff. Egg, activation of, 6ff. ; butyric acid, treatment of, 10; cortical layer of unfertilized, 10, 189; increased sensitiveness of unfertilized, to cytolytic agents, 163 ff.; production of twins from, 204 ff . Eggs, factors which determine time and place for the deposition of, 225; immunity of, to body extracts of same species, 142; varying susceptibility of, 144. Experiments, on hydroids, 217; on the egg of the frog, 216; on the mechanism of heliotropic reactions, 218; localization, 35.

Factors which determine time and place for the deposition of eggs, Heliotropic animals, 41; heliotropic reactions, experiments on the mechanism of, 218. Increased sensitiveness of unfertilized egg to cytolytic agents, 163 fl. Influence of membrane formation in causing the egg to develop, 150 ff. Light, change in Intensity of, 54 fl. effects of, 213; growth and, 213 photochemical action of, 30, 36 selection of intensity of, by animals 52 ff. Membrane of fertilization, 8, 148 flf.; membrane-forming substances, bases as, 134; membrane formation, emulsion theory of, 145, 147 flf. ; membrane formation, influence of, in causing egg to develop, 150 flf.

Monstrosities, artificial production of, in sea-urchins. 100 flf. Otoliths of ear and orientation to center of gravity of the earth, 57. Oxidations, and their relation to the egg after fertilization, 13, 157, 160 flf., 164; in their relation to life and death, 14, 15; eflfect of retarded, on poisonous salt action, 190 flf. substances, 39. Photosensitive surfaces, 39. Photosensitiveness, varying, in animals, Poisoning, acid, 18 ff.; salt, 186 flf. Poisonous action of distilled water on

constitution of the sea-water, 204 flf. Protective solution, 172. Purposeful character of reflexes, 66. Pycnopodia, 198. Pyrrhocoris, 16. Salt poisoning, 186 flf. ; diflfercnce of permeability of various membranes, 189 flf. Salts, antagonism of acids and, 179 flf.; antagonism of three salts. 182 ff.; cooperative action of, causing impermeability of egg membrane, 176 ff.; diffusion of, 177 ff. Selection, of intensity of light by animals, 52flf. ; natural, 50 fif.

Tubularia mesemhryanthemum, 95, 97. Tunicates, 92. Twin formation, 19, 205. Twins, production of, from egg, 204 ff. Twofold action of spermatozoon in