6. It may also be of interest to know that in helio­tropism the motions of the legs are automatically controlled by the chemical changes taking place in symmetrical elements of the retina. In order to prove this point we will turn to the phenomenon of galvano­tropism. The galvanic current forces certain animals to move in the direc­tion of one of the two electrodes just as the light forces the helio­tropic animals to move towards (or from) the source of light. The change in the concentra­tion of the ions at the boundary of the various organs, especially the nerves, determines the galvano­tropic reac­tions. When the shrimp Palæmonetes is put into a trough with dilute salt solu­tion through which a current of a certain intensity flows, the animal is compelled to move towards the anode.232 It can walk forwards, backwards, or sidewise. Here we can observe directly that the effect of the current consists in altering the tension of the muscles of the legs in such a way as to make it easy for the animal to move toward the anode and difficult to move toward the cathode. Thus if the current be sent sidewise through the animal, say from left to right (Fig. 45), the legs of the left side assume the flexor posi­tion, those of the right the extensor posi­tion. With this posi­tion of its legs the animal can easily move to the left, i. e., the anode, and only with difficulty to the right, i. e., the cathode. This change in the posi­tion of the legs occurs when the animal is not moving at all, thus showing that the galvano­tropic movements take place not because the animal intends to go to the anode, but that the animal goes to the anode because its legs are practically prevented by the galvanic current from working in any other way. This is exactly what happens in the helio­tropic motions of animals.233

To understand what happens when the current goes lengthwise through the body it should be stated that Palæmonetes uses the third, fourth, and fifth pairs of legs for its locomo­tion. The third pair pulls in the forward movement, and the fifth pair pushes. The fourth pair generally acts like the fifth, and requires no further atten­tion. If a current be sent through the animal longitudinally, from tail to head, and the strength be increased gradually, a change soon takes place in the posi­tion of the legs (Fig. 46). In the third pair the tension of the flexors predominates, in the fifth the tension of the extensors. The animal can thus move easily with the pulling of the third and the pushing of the fifth pairs of legs, that is to say, the current changes the tension of the muscles in such a way that the forward motion is rendered easy, the backward motion is difficult. Hence it can easily move toward the anode, but only with difficulty toward the cathode. If a current be sent through the animal in the opposite direc­tion, namely, from head to tail, the third pair of legs is extended, the fifth pair bent; that is, the third pair can push, and the fifth pair pull. The animal will thus move backward easily and forward with difficulty, and it is thus driven to the anode again.

The explanation which Loeb and Maxwell proposed for this influence of the current on the legs assumes that there are three groups of ganglion cells in the central nervous system of these animals which are oriented according to the three main axes of the body; (1) right-left and left-right, (2) backward, and (3) forward. It depends upon whether the ganglion cells or the nerve elements are in anelectro­tonus, which muscles are bent and which relaxed. It would lead us too far to recapitulate the theory in this place, and the reader who is interested in it is referred to Loeb and Maxwell’s paper.234 The importance of the observa­tions lies in the fact that they show that any element of will or choice on the part of the animal in these motions is eliminated, that the animal moves where its legs carry it, and not that the legs carry the animal where the latter “wishes” to go.

7. This may be the place to dispel an error which has sometimes crept into the discussion of the tropistic reac­tions of animals. It has been stated occasionally that it is the energy gradient and not the automatic orienta­tion of the animal by the light which makes the positively helio­tropic animal move towards the source of light and the negatively helio­tropic away from it. Thus the positively helio­tropic animal would be compelled to move towards the source of light as a consequence of the fact that the intensity of the light increases the more the nearer the animal approaches the source of light. If the source of light be the reflected sky-light the difference of intensity at both ends of a microscopic organism is so slight that it is beneath the limit capable of influencing the motions.

A simple experiment published by the writer in 1889 suffices to dispel the idea that the energy gradient determines the direc­tion of the motion of an animal in tropistic reac­tions. Let direct sunlight (S, Fig. 47) fall through the upper half of a window (w w) upon a table, and diffused daylight (D) through the lower half of the window on the same table. A test-tube a c is placed on the table in such a way that its long axis is at right angles to the plane of the window; and one half a b is in the direct sunlight, the other half in the shade. If at the beginning of the experi­ment the positively helio­tropic animals are in the direct sunlight at a, they promptly move toward the window, gathering at the window end c of the tube, although by so doing they go from the sunshine into the shade.235 This experi­ment is in harmony with our idea that the effect of light consists in turning the head of the animal and subsequently its whole body toward the source of light. By going from the strong light into the shade the reac­tion velocity in both eyes is diminished equally and hence there is no reason for the animal to change its orienta­tion, though its progressive motion may be stopped for an instant by the change. But at the boundary between sunlight and daylight a sudden change from strong to weak light occurs. If the energy gradient determined the direc­tion of the positively helio­tropic animal, the motion should stop at the boundary from strong to weak light, which may happen for an instant but which will not interfere with the progressive motion of the animal.

8. Graber had found that when animals are put into a trough covered half with blue and half with red glass, those that are “fond” of light go under the blue, those that are “fond” of darkness go under the red glass. The writer pointed out that this result should be expected on the basis of his theory of helio­tropism, if the assump­tion be correct that the red light is considerably less efficient than light which goes through blue glass (such glass also allows green rays to go through). The botanists had already shown that red glass is impermeable for the rays which cause helio­tropic reac­tions of plants, and the writer was able to show the same for the helio­tropic reac­tions of animals. Red glass acts, therefore, almost like an opaque body for these animals.

A closer examination of the most efficient rays for the helio­tropic reac­tions of different organisms has revealed the fact that for some organisms a region in the blue λ = 460–490 µµ, for others a region in the yellowish-green, near about λ = 520–530 µµ is the most efficient.236 For many plants and for some animals, like Eudendrium and the larvæ of the worm Arenicola, a region in the blue is most efficient; for certain, if not most, animals a region in the yellow-green is most efficient. Among unicellular green algæ, Chlamydomonas, has its maximal efficiency in the yellowish-green and Euglena in the blue. According to observa­tions by Mast, some green unicellular organisms like Pandorina, Eudorina, and Spondylomorum seem to behave more like Chlamydomonas, while certain others behave more like Euglena.237 Wasteneys and the writer suggested that there are two groups of helio­tropic substances, one with a maximum of photo­sensitiveness in the blue, the other in the yellowish-green; and that the latter group may or may not be related or identical with the visual purple which is most rapidly bleached by light of a wave length near λ = 520–530 µµ.

The ophthalmologist Hess238 has utilized the helio­tropic reac­tions of animals in an attempt to prove that all animals from the lowest invertebrates up to the fishes inclusive suffer from total colour-blindness. This statement was based on the observa­tion that for most positively helio­tropic animals the region in the yellowish-green near λ = 520 µµ seems the most efficient. Since this region of the spectrum appears also as the brightest to a totally colour-blind man, he concluded that all these animals are totally colour-blind. There is no reason why helio­tropic reac­tions should be used as an indicator for colour sensa­tions; if totally colour-blind human beings were possessed of an irresistible impulse to run into a flame Hess’s assump­tion might be considered, but no such phenomenon exists in colour-blind man. Moreover, v. Frisch239 has shown by experi­ments on the influence of the background on the coloura­tion of fish as well as by experi­ments on bees and on Daphnia that the reac­tions of these animals to light of different wave-lengths indicate different effects besides those of mere intensity. Thus v. Frisch could train bees to go to a blue piece of cardboard distributed among many cardboards of different shades of grey. Bees thus trained would alight on any blue object even if it contained no food. It would be impossible to do this with totally colour-blind organisms.

9. Heliotropic reactions play a great rôle in the preserva­tion of individuals as well as of species. In order to understand this rôle it must be stated that the photo­sensitive substances appear often only under certain condi­tions and that their effect is inhibited under other condi­tions. Thus among ants the winged males and females alone show positive helio­tropism,240 while the wingless workers are free from this reac­tion. This positive helio­tropism becomes violent at the time of the nuptial flight and this phenomenon itself seems to be a helio­tropic phenomenon since it takes place in the direc­tion of the light. When the queen founds her nest she loses her wings and becomes negatively helio­tropic again. Kellogg241 has shown that the nuptial flight of the bees is also a purely helio­tropic phenomenon. When a part of the hive remote from the entrance is illuminated the bees rush to the light and can thus be prevented from swarming. These phenomena suggest that the presence of some substance secreted by the sex glands may cause the intensifica­tion of the helio­tropism which leads to the nuptial flight.

In certain species of Daphnia, fresh-water copepods, and of Volvox, a trace of CO₂ suffices to make negatively helio­tropic or indifferent specimens violently positively helio­tropic.242 Certain forms of marine copepods and the larvæ of Polygordius can be made positively helio­tropic by lowering the temperature243 and the larvæ of the barnacle can be made negatively helio­tropic by strong light.244 It is quite possible that a change in the sense of helio­tropism by temperature and light is to some extent at least responsible for the periodic depth migra­tions of helio­tropic animals. Many if not all positively helio­tropic animals can be made negatively helio­tropic by exposure to ultraviolet light.245

A most interesting example of the rôle of helio­tropism in the preserva­tion of a species is shown in the caterpillars of Porthesia chrysorrhœa. The butterfly lays its eggs upon a shrub. The larvæ hatch late in the fall and hibernate in a nest on the shrub, as a rule not far from the ground. As soon as the temperature reaches a certain height, they leave the nest; under natural condi­tions, this happens in the spring when the first leaves have begun to form on the shrub. (The larvæ can, however, be induced to leave the nest at any time in the winter provided the temperature is raised sufficiently.) After leaving the nest, they crawl directly upward on the shrub where they find the leaves on which they feed. Should the caterpillars move down the shrub, they would starve, but this they never do, always crawling upward to where they find their food. What gives the caterpillar this never-failing certainty which saves its life, and for which a human being might envy the little larva? Is it a dim recollec­tion of experiences of former genera­tions? It can be shown that it is the light reflected from the sky which guides the animal upward. When we put these animals into a horizontal test-tube in a room, they all crawl toward the window, or toward a lamp; the animal is positively helio­tropic. It is this positive helio­tropism which makes them move upward where they find their food, when the mild air of the spring calls them forth from their nest. At the top of the branch, they come in contact with a leaf, and chemical or tactile influences set the mandibles of the young caterpillar into activity. If we put these larvæ into closed test-tubes which lie with their longitudinal axes at right angles to the window, they will all migrate to the window end, where they stay and starve, even if their favourite leaves are close behind them. They are slaves of the light.

The few young leaves on top of a twig are quickly eaten by the caterpillar. The light, which saved its life by making it creep upward where it finds food, would cause it to starve could it not free itself from the bondage of positive helio­tropism. The animal, after having eaten, is no longer a slave of the light, but can and does creep downward. It can be shown that a caterpillar, after having been fed, loses its positive helio­tropism almost completely and permanently. If we submit unfed and fed caterpillars of the same nest contained in two different test-tubes to the same artificial or natural source of light, the unfed will creep to the light and stay there until they die, while those that have eaten will pay little or no atten­tion to the light. Their sensitiveness to light has disappeared; after having eaten they become independent of light and can creep in any direc­tion. The restlessness which accompanies the condi­tion of hunger makes the animal creep downward—which is the only direc­tion open to it—where it finds new young leaves on which it can feed. The wonderful hereditary instinct, upon which the life of the animal depends, is its positive helio­tropism in the unfed condi­tion and its loss of this helio­tropism after having eaten. The latter phenomenon is in harmony with the experi­ments which show that the helio­tropism of certain species of Daphnia disappears when the water becomes neutral.

And finally it may be pointed out that the majority of green plants could not exist if their stems were not positively, their roots negatively, helio­tropic. It is the positive helio­tropism which makes the top grow toward the light, which enables the leaves to get the light necessary for assimila­tion, and the roots to grow into the soil where they find the water and nutritive salts.

10. While we do not wish to deal here with the different tropisms it should be stated that aside from helio­tropism, chemotropism as well as stereotropism play the most essential rôle in the so-called instinctive ac­tions of animals. It is a problem of orienta­tion by the diffusion of molecules from a centre when a male butterfly is deviated from its flight and alights on the wooden box in which is enclosed a female of the same species. We have already alluded to certain phenomena of chemotropism in Chapter IV. Certain organisms have a tendency to bring their bodies as much as possible on all sides in contact with solid bodies; thus the butterfly Amphipyra, which is a fast runner, will come to rest under a glass plate when the plate is put high enough above the ground so that it touches the back of the butterfly. The animals which live under stones or underground or in caves are as a rule both negatively helio­tropic and positively stereotropic. Their tropisms predestine or force them into the life they lead.

The sensitive area which forms the basis of tropisms is as a rule developed not in the whole organism but only in certain segments of the body. Thus the eyes are located in the head. But when the action of one segment becomes overpowering the whole organism follows the segment. It has been customary among physiologists to speak of reflexes in such cases. Thus, e. g., the arms of the male frog develop a powerful positive stereotropism on their ventral surface during the spawning season. It would avoid confusion to realize that there is nothing gained in applying to this tropism the meaningless term “reflex”; it is better to call them tropisms since the organism as a whole is involved. If necessary we might speak of segmental tropisms. The act of seeking the female as well as that of cohabita­tion are in many cases combina­tions of chemotropism and stereotropism. The development of these tropisms depends upon the presence of certain specific substances in the body, a fact emphasized already in the case of helio­tropism. In case of the development of the segmental stereotropism of the male frog at the time of spawning it has been shown that it depends on an internal secre­tion from the testes.

It has been suggested by some authors that the tropistic reac­tions are determined by some feeling or emotion on the part of the organism. We have no means of judging the emotions of lower animals (except by “intui­tion”). The writer suggested in 1899 in his book on brain physi­ology that emotions may be determined by specific substances which also determine the tropistic reac­tion (as well as phenomena of organ forma­tion, although this latter phenomenon has nothing to do with the subject of instincts); and the excellent work of Cannon246 has shown the rôle of adrenalin in the expression of fear. It is, therefore, both unwarranted and unnecessary to state that hypothetical emotions determine the tropistic reac­tions.

CHAPTER XI

THE INFLUENCE OF ENVIRONMENT

1. The term environ­ment in rela­tion to an organism may easily assume a mystic rôle if we assume that it can modify the organisms so that they become adapted to its peculiarities. Such ideas are difficult to comprehend from a physico­chemical viewpoint, according to which environ­ment cannot affect the living organism and non-living matter in essentially different ways. Of course we know that proteins will as a rule coagulate at temperatures far below the boiling point of water and that no life is conceivable for any length of time at temperatures above 100° C., but heat coagula­tion of proteins occurs as well in the test-tube as in the living organism. If we substitute for the indefinite term environ­ment the individual physical and chemical forces which constitute environ­ment it is possible to show that the influence of each of these forces upon the organism finds its expression in simple physico­chemical laws and that there is no need to introduce any other considera­tions.

We select for our discussion first the most influential of external condi­tions, namely temperature. The reader knows that there is a lower as well as an upper temperature limit for life. Setchell has ascertained that in hot springs whose temperature is 43° C., or above, no animals or green alga are found.247 In hot springs whose temperature is above 43° he found only the Cyanophyceæ, whose structure is more closely related to that of the bacteria than to that of the algæ, inasmuch as they have neither definitely differentiated nuclei nor chromophores. The highest temperature at which Cyanophyceæ occurred was 63° C. Not all the Cyanophyceæ were able to stand temperatures above 43° C., but only a few species. The other Cyanophyceæ were found at a temperature below 40° C., and were no more able to stand higher temperatures than the real algæ or animals. The Cyanophyceæ of the hot springs were as a rule killed by a temperature of 73°. From this we must conclude that they contain proteins whose coagula­tion temperature lies above that of animals and green plants, and may be as high as 73°. Among the fungi many forms can resist a temperature above 43° or 45°; the spores can generally stand a higher temperature than the vegetative organs. Duclaux found that certain bacilli (Tyrothrix) found in cheese are killed in one minute at a temperature of from 80° to 90°; while for the spores of the same bacillus a temperature of from 105° to 120° was required.248

Duclaux has called atten­tion to a fact which is of importance for the investiga­tion of the upper temperature limit for the life of organisms. According to this author it is erroneous to speak of a definite temperature as a fatal one; instead we must speak of a deadly temperature zone. This is due to the fact that the length of time which an organism is exposed to a higher temperature is of importance. Duclaux quotes as an example a series of experi­ments by Christen on the spores of soil and hay bacilli. The spores were exposed to a stream of steam and the time determined which was required at the various temperatures to kill the spores.

In warm-blooded animals 45° is generally considered a temperature at which death occurs in a few minutes; but a temperature of 44°, 43°, or 42° is also to be considered fatal with this difference only, that it takes a longer time to bring about death. This fact is to be considered in the treatment of fever.

It is generally held that death in these cases is due to an irreversible heat coagula­tion of proteins. According to Duclaux, it can be directly observed in micro-organisms that in the fatal temperature zone the normally homogeneous, or finely granulated, protoplasm is filled with thick, irregularly arranged bodies, and this is the optical expression of coagula­tion. The fact that the upper temperature limit differs so widely in different forms is explained by Duclaux through differences in the coagula­tion temperature of the various proteins. It is, e. g. known that the coagula­tion temperature varies with the amount of water of the colloid. According to Cramer, the mycelium of Penicillium contains 87.6 water to 12.4 dry matter, while the spores have 38.9 water and 61.1 dry substance. This may explain why the mycelium is killed at a lower temperature than the spores. According to Chevreul, with an increase in the amount of water, the coagula­tion temperature of albuminoids decreases. The reac­tion of the protoplasm influences the temperature of coagula­tion, inasmuch as it is lower when the reac­tion is acid, higher when the reac­tion is alkaline. The experi­ments of Pauli show also a marked influence of salts upon the temperature of coagula­tion of colloids.

The process of heat coagula­tion of colloids is also a func­tion of time. If the exposure to high temperature is not sufficiently long, only part of the colloid coagulates; in this case an organism may again recover.

Inside of these upper and lower temperature limits we find that life phenomena are influenced by temperature in such a way that their rate is about doubled for an increase of the temperature of 10° C., and that this temperature coefficient for 10°, Q₁₀, very often steadily diminishes from the lower to the higher temperature; so that near the lower temperature limit it becomes often considerably greater than 2 and near the higher temperature limit it becomes very often less than 2.249 This influence of temperature is so general that we are bound to associate it with an equally general feature of life phenomena; and such a feature would be most likely the chemical reac­tions. It is known through the work of Berthelot, van’t Hoff, and Arrhenius that the temperature coefficient for the velocity of chemical reac­tions is also generally of about the same order of magnitude; namely ≧2 for a difference of 10°. In chemical reac­tions there is also a tendency for Q₁₀ to become larger for lower temperature, and coefficients of Q₁₀ about 5 or 6 have repeatedly been found for purely chemical reac­tions between 0° and 10°, e. g., for the inversion of cane sugar by the hydrogen ion. The temperature coefficient for the reac­tion velocity of ferments shows the same diminu­tion of Q₁₀ with rising temperature which is also noticed in most life phenomena. Thus Van Slyke and Cullen250 found that the reac­tion rate of the enzyme urease “is nearly doubled by every 10° rise in temperature between 10° and 50°. Within this range the temperature coefficient is nearly constant and averages 1.91. From O° to 10° it is 2.80, from 50° to 60° it is only 1.09. The optimum is at about 55°.” The rapid fall of the temperature coefficient for enzyme action at the upper temperature limit has been ascribed by Tammann to a progressive destruc­tion of the active mass of enzyme by the higher temperature (by hydrolysis). This will, however, not account for the high value of the coefficient near the lower limit. But is it not imaginable that at low temperature an aggrega­tion of the enzyme particles exists which is also equivalent to a diminu­tion of the active mass of the enzyme and that this aggrega­tion is gradually dispersed by the rising temperature? This would account for the fact that at a temperature near 0°C life phenomena stop because the enzymes are all in a state of aggrega­tion or gela­tion; that then more and more are dissolved and the rate of chemical reac­tion increases since the mass of enzyme particles increases until all the enzyme molecules are dissolved or rendered active. Under this assump­tion three processes are superposed in the varia­tion of the value of Q₁₀ with temperature: (1) the supposed increase in the number of available ferment molecules with increasing temperature near the lower temperature limit; (2) the temperature coefficient of the reac­tion velocity which is nearly = 2 for 10°C.; (3) the diminu­tion of the number of available ferment molecules by hydrolysis or some other action of the increasing temperature. This latter is noticeable near the upper temperature limit. The reason that 1 and 3 interfere more strongly in life phenomena than in the chemical reac­tions of crystalloid substances may possibly be accounted for by the fact that the enzymes and most of the constituents of living matter are colloidal, i. e., consist of particles of a considerably greater order of magnitude than the molecules of crystalloids.251

We will now show the rôle of the temperature coefficient upon phenomena of development. F. R. Lillie and Knowlton252 first determined the influence of temperature upon the development of the egg of the frog and showed that it was of the same nature as that of a chemical reac­tion. These experi­ments were repeated a year later by O. Hertwig.253

The time required for the eggs to reach definite stages was measured for different temperatures and it was found that the temperature coefficient Q₁₀ between 2.5° and 6° was equal to 10 or more; between 6° and 15° it was between 2.6 and 4.5; between 10° and 20° it was 2.9 to 3.3, and between 20° and 24° it was between 1.4 and 2.0. To anybody who has worked on this problem it is obvious that no exact figures can be obtained in this way, since the point when a certain stage of development is reached is not so sharply defined as to exclude a certain latitude of arbitrariness. The writer found that very exact figures can be obtained on the influence of temperature upon development of the sea-urchin egg by measuring the time from insemina­tion to the first cell division. Such experi­ments were carried out in a cold-water form Strongylo­centrotus purpuratus and a form living in warmer water, Arbacia.254 The figures on Arbacia have been verified by different observers in different years.

TABLE X

Influence of Temperature upon the Time (in Minutes) Required From Insemination to the First Cell Division

These figures permitted the determina­tion of the temperature coefficients Q₁₀ with a sufficient degree of accuracy (see next table). It seemed of importance to attempt to decide what the chemical reac­tion underlying these reac­tion velocities is (if it is a chemical reac­tion). Loeb and Wasteneys255 investigated the temperature coefficient for the rate of oxida­tions in the newly fertilized egg of Arbacia and found that the temperature coefficient Q₁₀ for that process does not vary in the same way as the temperature coefficient for cell division.

TABLE XI

Temperature Coefficients Q₁₀ for the Rate of Segmentation and Oxidations in the Eggs of Strongylocentrotus AND Arbacia

It is obvious that the temperature coefficient of the rate of oxida­tions is remarkably constant, about 2 for 10°, for various temperatures and does not show the varia­tion from 7 or more to 2.2 for Q₁₀ for the rate of segmenta­tion.

Kanitz256 has shown that in a graph in which the logarithms of the segmenta­tion velocities are drawn as ordinates and the temperatures as abscissæ the logarithms form two straight lines which are joined at an angle. According to the law of van’t Hoff and Arrhenius concerning the influence of temperature upon velocities of chemical reac­tions the logarithms should lie in a straight line. We are dealing therefore in these cases with two exponential curves, one representing in Arbacia the interval 7–13° and the second from 13–26°; in Strongylo­centrotus between 3–9° and 9–20°.

It was found in these experi­ments that if measurements of the Q₁₀ of later stages of development are attempted the varia­tions due to unavoidable difficulties become too great to permit an equal degree of reliability in the determina­tions.

The vast importance of this influence of temperature upon the rate of development is seen in the fact that in addi­tion to the food supply the rate of the maturing of plants and animals depends on this factor.

2. This influence of temperature upon development has been used to find the condi­tions determining fluctuating varia­tion. The reader knows that by this expression are understood the differences between individuals of a pure strain or breed. These varia­tions are not inherited, a fact contrary to the idea of Darwin, who assumed that by the selec­tion of extreme cases of fluctuating varia­tion new varieties could develop. What is the basis of this fluctuating varia­tion? The writer concluded that if fluctuating varia­tions were due to a slight varia­tion in the quantity of a specific substance—in some cases an enzyme—required for the forma­tion of a hereditary character, the temperature coefficient might be used to test the idea. We have just seen that the time required from insemina­tion until the cell division of the first egg occurs is very sharply defined for each temperature. If a large number e. g. one hundred or more eggs are under observa­tion simultaneously in a microscopic field it can be seen that they do not all segment at the same time but in succession; this is the expression of fluctuating varia­tion. Miss Chamberlain and the writer have measured the time which elapses between the moment the first egg of such a group segments and the moment the last egg begins its segmenta­tion, and found that this latitude of varia­tion is also very definite for each temperature, and that its temperature coefficient is for each interval of 10° practically identical with the temperature coefficient of the segmenta­tion for the same interval.257 The slight devia­tions are practically all in the same sense and accounted for by a slight deficiency in the nature of the experi­ments. The two following tables give the latitude of varia­tions for different temperatures for the first segmenta­tion in Arbacia and the temperature coefficient for this latitude and the rate of segmenta­tion. These two latter coefficients are practically identical.

TABLE XII

TABLE XIII

If we assume that the temperature coefficient for the segmenta­tion of the egg is that of a chemical reac­tion (other than oxida­tion) underlying the process of segmenta­tion, the fluctuating varia­tion in the time of the segmenta­tions of the various eggs fertilized at the same time is due to the fact that the mass of the enzyme controlling that reac­tion varies within definite limits in different eggs. The first egg segmenting at a given temperature has the maximal, the last egg segmenting has the minimal mass of enzyme. It should be added that the time of the first segmenta­tion is determined by the cytoplasm and is not a Mendelian character, as was stated in a previous chapter.

3. The point of importance to us is that the influence of temperature upon the organism is so constant that if disturbing factors are removed it would be possible to use the time from insemina­tion to the first segmenta­tion of an egg of Arbacia as a thermometer on the basis of the table on page 295.

Facts of this character should dispose of the idea that the organism as a whole does not react with that degree of machine-like precision which we find in the realm of physics and chemistry. Such an idea could only arise from the fact that biologists have not been in the habit of looking for quantitative laws, chiefly, perhaps, because the difficulties due to disturbing secondary factors were too great. The worker in physics knows that in order to discover the laws of a phenomenon all the disturbing factors which might influence the result must first be removed. When the biologist works with an organism as a whole he is rarely able to accomplish this since the various disturbing influences, being inseparable from the life of the organism, can often not be entirely removed. In this case the biologist must look for an organism in which by chance this elimina­tion of secondary condi­tions is possible. The following example may serve as an illustra­tion of this rather important point in biological work. Although all normal human beings have about the same temperature, yet if the heart-beats of a large number of healthy human beings are measured the rate is found to vary enormously. Thus v. Körösy found among soldiers under the most favourable and most constant condi­tions of observa­tions—the soldiers were examined early in the morning before rising—varia­tions in the rate of heart-beat between 42 and 108. In view of this fact, those opposed to the idea that the organism as a whole obeys purely physico­chemical laws might find it preposterous to imagine that the rate of heart-beat could be used as a thermometer. Yet if we observe the influence of temperature on the rate of the heart-beat of a large number of embryos of the fish Fundulus, while the embryos are still in the egg, we find that at the same temperature each heart beats at the same rate, the devia­tions being only slight and such as the fluctuating varia­tions would demand.258 This constancy is so great that the rate of heart-beat of these embryos could in fact be used as a rough thermometer. The influence of temperature upon the rate of heart-beat is completely reversible so that when we measure the rate for increasing as well as for decreasing temperatures we get approximately the same values as the following table shows.

TABLE XIV

Why does each embryo have the same rate of heart-beat at the same temperature in contradistinc­tion to the enormous variability of the same rate in man? The answer is, on account of the elimina­tion of all secondary disturbing factors. In the embryo of Fundulus the heart-beat is a func­tion almost if not exclusively of two variables, the mass of enzymes for the chemical reac­tions underlying the heart-beat and the temperature. By inheritance the mass of enzymes is approximately the same and in this way all the embryos beat at the same rate (within the limits of the fluctuating varia­tion) at the same temperature. This identity exists, however, only as long as the embryo is relatively quiet in the egg. As soon as the embryo begins to move this equality disappears since the motion influences the heart-beat and the motility of different embryos differs.

In man the number of disturbing factors is so great that no equality of the rate for the same temperature can be expected. Differences in emotions or the internal secre­tions following the emotions, differences in previous diseases and their after-effects, differences in metabolism, differences in the use of narcotics or drugs, and differences in activity are only some of the number of variables which enter.

4. As stated above the temperature influences practically all life phenomena in a similar characteristic way, e. g., the produc­tion of CO₂ in seeds259 and the assimila­tion of CO₂ by green plants.260 The writer would not be surprised if even the aberra­tions in the colour of butterflies under the influence of temperature turned out to be connected with the temperature coefficient. The experi­ments of Dorfmeister, Weismann, Merrifield, Standfuss, and Fischer, on seasonal dimorphism and the aberra­tion of colour in butterflies have so often been discussed in biological literature that a short reference to them will suffice. By seasonal dimorphism is meant the fact that species may appear at different seasons of the year in a somewhat different form or colour. Vanessa prorsa is the summer form, Vanessa levana the winter form of the same species. By keeping the pupæ of Vanessa prorsa several weeks at a temperature of from 0° to 1° Weismann succeeded in obtaining from the summer chrysalids specimens which resembled the winter variety, Vanessa levana.

If we wish to get a clear understanding of the causes of varia­tion in the colour and pattern of butterflies, we must direct our atten­tion to the experi­ments of Fischer, who worked with more extreme temperatures than his predecessors, and found that almost identical aberra­tions of colour could be produced by both extremely high and extremely low temperatures. This can be seen clearly from the following tabulated results of his observa­tions. At the head of each column the reader finds the temperature to which Fischer submitted the pupæ, and in the vertical column below are found the varieties that were produced. In the vertical column A are given the normal forms:

TABLE XV

The reader will notice that the aberra­tions produced at a very low temperature (from 0° to -20°C.) are absolutely identical with the aberra­tions produced by exposing the pupæ to extremely high temperatures (42° to 46°C.). Moreover, the aberra­tions produced by a moderately low temperature (from 0° to 10°C.) are identical with the aberra­tions produced by a moderately high temperature (36° to 41°C.).

From these observa­tions Fischer concludes that it is erroneous to speak of a specific effect of high and of low temperatures, but that there must be a common cause for the aberra­tion found at the high as well as at the low temperature limits. This cause he seems to find in the inhibiting effects of extreme temperatures upon development.

If we try to analyse such results as Fischer’s from a physico­chemical point of view, we must realize that what we call life consists of a series of chemical reac­tions, which are connected in a catenary way; inasmuch as one reac­tion or group of reac­tions (a) (e. g., hydrolyses) causes or furnishes the material for a second reac­tion or group of reac­tions (b) (e. g., oxida­tions). We know that the temperature coefficient for physio­logical processes varies slightly at various parts of the scale; as a rule it is higher near 0° and lower near 30°. But we know also that the temperature coefficients do not vary equally for the various physio­logical processes. It is, therefore, to be expected that the temperature coefficients for the group of reac­tions of the type (a) will not be identical through the whole scale with the temperature coefficients for the reac­tions of the type (b). If therefore a certain substance is formed at the normal temperature of the animal in such quantities as are needed for the catenary reac­tion (b), it is not to be expected that this same perfect balance will be maintained for extremely high or extremely low temperatures; it is more probable that one group of reac­tions will exceed the other and thus produce aberrant chemical effects, which may underlie the colour aberra­tions observed by Fischer and other experi­menters.

It is important to notice that Fischer was also able to produce aberra­tions through the applica­tion of narcotics. Wolfgang Ostwald has produced experimentally, through varia­tion of temperature, dimorphism of form in Daphnia.

5. Next or equal in importance with the temperature is the nature of the medium in which the cells are living.

It has often been pointed out that the marine animals and the cells of the body of metazoic animals are surrounded by a medium of similar constitu­tion, the sea water and the blood or lymph, both media being salt solu­tions differing in concentra­tion but containing the three salts NaCl, KCl, and CaCl₂ in about the same relative concentra­tion, namely 100 molecules NaCl : 2.2 molecules of KCl : 1.5 molecules of CaCl₂. This has suggested to some authors the poetical dream that our home was once the ocean, but we cannot test the idea since unfortunately we cannot experi­ment with the past. Plants, unicellular fresh-water algæ, and bacteria do not demand such a medium for their existence.

Herbst had shown that when sea-urchin larvæ were raised in a medium in which only one of the constituents of the sea water was lacking (not only NaCl, KCl, or CaCl₂, but also Na₂SO₄, NaHCO₃, or Na₂HPO₄), the eggs could not develop into plutei; from which he concluded that every constituent of the sea water was necessary. This would indicate a case of extreme adapta­tion to all the minutiæ of the external medium.

Experiments on a much more favourable animal for this purpose, namely, the eggs of the marine fish Fundulus, gave altogether different results. The eggs of this marine fish develop naturally in sea water but they develop just as well in fresh or in distilled water, and the young fish when they are made to hatch in distilled water will continue to live in this medium. This proves that these eggs require none of the salts of the sea water for their development. When these eggs are put immediately after fertiliza­tion into a pure solu­tion of NaCl of that concentra­tion in which this salt exists in the sea water practically all the eggs die without forming an embryo; but if a small quantity of CaCl₂ is added every egg is able to form one, and these embryos will develop into fish and the latter will hatch. This led the writer to the conclusion that these fish (and perhaps marine animals in general) need the Ca of the sea water only to counteract the injurious effects which a pure NaCl solu­tion has if it is present in too high a concentra­tion.261 When we raise the eggs in a pure NaCl solu­tion of a concentra­tion ≦m/8 practically every egg will develop; and even in a m/4 or ³⁄₈ m many or some eggs will form embryos without adding Ca; it may be that a trace of Ca present in the membrane of the egg may suffice to counter-balance the injurious action of a weak salt solu­tion.

The concentra­tion of the NaCl in the sea water at Woods Hole (where these experi­ments were made) is about m/2, and as soon as this concentra­tion of NaCl is reached the eggs are all killed as a rule before they can form an embryo, unless a small but definite amount of Ca is added. It was found that the eggs can be raised in much higher concentra­tions of NaCl, but in that case more Ca must be added. The following table gives the minimal amount of CaCl₂ which must be added in order to allow fifty per cent. of the eggs to form embryos. (The eggs were put into the solu­tion an hour or two after fertiliza­tion.)

TABLE XVI

This indicates that the quantity of CaCl₂ required to counteract the injurious effects of a pure solu­tion of NaCl increases approximately in propor­tion to the square of the concentra­tion of the NaCl solu­tion.262 The reader will notice that the eggs can survive and develop in a solu­tion of three times the concentra­tion of sea water, provided enough Ca is added.

It was found also that not only Ca but a large number of other bivalent metals were able to counteract the injurious action of an excessive NaCl solu­tion; namely Mg, Sr, Ba, Mn, Co, Zn, Pb, and Fe;263 only Hg and Cu could not be used since they are themselves too toxic. The antagonistic efficiency of the bivalent cations other than Ca was, however, smaller than that of Ca. The following table gives the highest concentra­tion of NaCl solu­tion in which the newly fertilized eggs of Fundulus can still form an embryo.264

50 c.c. ¹⁰⁄₈ m NaCl+4 c.c. m/1 MgCl₂
50 c.c. ¹⁴⁄₈ m NaCl+1 c.c. m/1 CaCl₂
50 c.c. ¹¹⁄₈ m NaCl+1 c.c. m/1 SrCl₂
50 c.c.  ⁷⁄₈ m NaCl+1 c.c. m/1 BaCl₂

On the other hand it was seen that in all the chlorides with a univalent cation, LiCl, KCl, RbCl, CsCl, NH₄Cl, the eggs could form embryos up to a certain concentra­tion of the salt; but that this concentra­tion could be raised by the addi­tion of Ca.

TABLE XVII

Concentrations at which the Eggs no longer Are Able to Form Embryos

In short it turned out that the injurious action of the pure solu­tion of any chloride (or any other anion) with a univalent metal could be counteracted to a considerable extent by the addi­tion of small quantities of a salt with a bivalent metal. It was also found in the early experi­ments of the writer that the bivalent or polyvalent anions had no such antagonistic effect upon the injurious action of the salts with a univalent cation.

We therefore see that what at first sight appeared in the experi­ments of Herbst a necessity, namely, the presence of each constituent of the sea water, turns out as a special case of a more general law; the salts with univalent ions are injurious if their concentra­tion exceeds a certain limit and this injurious action is diminished by a trace of a salt with a bivalent cation.

Why was it not possible to prove this fact for the eggs of the sea urchin? Before we answer this ques­tion, we wish to enter upon the discussion of the nature of the injurious action of a pure NaCl solu­tion of a certain concentra­tion and of the annihila­tion of this action by the addi­tion of a small quantity of Ca. The writer suggested in 1905 that the injurious action of a pure NaCl solu­tion consisted in rendering the membrane of the egg permeable for NaCl, whereby the germ inside the membrane is killed; while the addi­tion of a small amount of Ca (or any other bivalent metal) prevents the diffusion of Na into the egg,265 possibly, as T. B. Robertson266 suggested, by forming a precipitate with some constituent of the membrane, whereby the latter becomes more impermeable. The correctness of this idea can be demonstrated in the following way. When eggs of Fundulus, which are three or four days old and contain an embryo, are put into a test-tube containing 3 m NaCl they will float on this solu­tion for about three or four hours; after that they will sink to the bottom. Before this happens the egg will shrink and when it ceases to float the embryo is usually dead. This is intelligible on the assump­tion that the NaCl solu­tion entered the egg, increased its specific gravity so that it could not float any longer and killed the embryo. When we add, however, 1 c.c. ¹⁰⁄₈ m CaCl₂ to 50 c.c. 3 m NaCl the eggs will float, the heart will continue to beat normally and the embryo will continue to develop for three days or more, because the calcium prevents the NaCl from entering into the egg.267 For if we put a newly hatched embryo into 50 c.c. NaCl+1 c.c. ¹⁰⁄₈ m CaCl₂ it will die almost instantly; hence the membrane must have acted for three or more days as a shield which prevented the NaCl from diffusing into the egg in the presence of CaCl₂.