| _Difference of Intensity between the Two Lights_
_Number +---------+---------+---------+---------+---------+---------
of | 0 | 8-1/3 | 16-2/3 | 25 | 33-1/3 | 50
Glowers_|per cent.|per cent.|per cent.|per cent.|per cent.|per cent.
---------+---------+---------+---------+---------+---------+---------
| _Deflection in Degrees_
1 | -0.55 | -2.32 | -5.27 | -9.04 | -11.86 | -19.46
2 | -0.10 | -3.05 | -6.12 | -8.55 | -11.92 | -22.28
3 | +0.45 | -2.60 | -5.65 | -8.73 | -13.15 | -20.52
4 | -0.025 | -2.98 | -6.60 | -9.66 | -11.76 | -19.88
5 | -0.225 | -2.92 | -5.125 | -8.30 | -10.92 | -19.28
---------+---------+---------+---------+---------+---------+---------
Average | -0.09 | -2.77 | -5.75 | -8.86 | -11.92 | -20.28
---------+---------+---------+---------+---------+---------+---------
Such constancy of quantitative results is only possible where we are
dealing with purely physicochemical phenomena or where life phenomena
are unequivocally determined by purely physicochemical conditions.
5. It seems difficult for some biologists, even with the validity of
the Bunsen-Roscoe law proven, to imagine that the movements of the
animals under the influence of light are not voluntary (or not dictated
by the mysterious “trial and error” method of Jennings).[231] But
one wonders how it is possible on such an assumption to account for
the fact that the angle of deflection of the larva of the fly when
under the influence of two lights of different intensities should be
always the same for a given difference in intensity; or why the time
for curvature in _Eudendrium_ should vary inversely with the intensity
of illumination. It is, however, possible to complete the case for
the purely physicochemical analysis of these instincts. John Hays
Hammond, Jr., has succeeded in constructing heliotropic machines which
in the dark follow a lantern very much in the manner of a positively
heliotropic animal. The eyes of this heliotropic machine consist of two
lenses in whose focus is situated the “retina” consisting of selenium
wire. The two eyes are separated from each other by a projecting piece
of wood which represents the nose and allows one eye to receive light
while the other is shaded. The galvanic resistance of selenium is
altered by light; and when one selenium wire is shaded while the other
is illuminated, the electric energy (supplied by batteries inside the
machine) which makes the wheels turn (these take the place of the legs
of the normal animal) no longer flows symmetrically to the steering
wheel, and the machine turns towards the light. In this way the
machine follows a lantern in a dark room in a way similar to that of a
positively heliotropic animal. Here we have a model of the heliotropic
animal whose purely mechanistic character is beyond suspicion, and we
may be sure that it is not “fondness” for light or for brightness nor
will-power nor a method of “trial and error” which makes the machine
follow the light.
[231] According to this theory the animal is not directly oriented by
the outside force, _e. g._ the light, but selects among its random
movements the one which is most “suited” and keeps on moving in this
direction. This idea is untenable for most if not all the cases of
tropisms and has been refuted by practically all the workers in this
field, _e. g._, Parker and his pupils, Bohn, H. B. Torrey, Holmes,
Bancroft, Ewald, and others. It is only upheld by Jennings and Mast;
and is accepted among those to whom the idea of a physicochemical
explanation of life phenomena does not appeal. Torrey and Bancroft
(for the literature the reader is referred to Bancroft’s paper, _Jour.
Exper. Zoöl._, 1913, xv., 383) have shown directly that the theory of
trial and error is not even correct for the organism for which Jennings
has developed this idea; namely _Euglena_.
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]
[232] Loeb, J., and Maxwell, S. S., _Arch. f. d. ges. Physiol._, 1896,
lxiii., 121.
[233] That the mechanisms by which heliotropic and galvanotropic
orientation is brought about are identical was shown by Bancroft in
_Euglena_ (Bancroft, _loc. cit._).
[Illustration: FIG. 45]
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.
[Illustration: FIG. 46]
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.
[234] Loeb, J., and Maxwell, S. S., _Arch. f. d. ges. Physiol._, 1896,
lxiii., 121.
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.
[Illustration: FIG. 47]
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.
[235] Loeb, J., _Dynamics of Living Matter_, p. 126.
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.
[236] Loeb, J., and Maxwell, S. S., _Univ. Cal. Pub._, 1910,
_Physiol._, iii., 195; Loeb and Wasteneys, _Proc. Nat. Acad. Sc._,
1915, i., 44; _Science_, 1915, xli., 328; _Jour. Exper. Zoöl._, 1915,
xix., 23; 1916, xx., 217.
[237] Mast, S. O., _Proc. Nat. Acad. Sc._, 1915, i., 622.
The ophthalmologist Hess[238] 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. Frisch[239] 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.
[238] Hess, C., “Gesichtssinn,” _Winterstein’s Handb. d. vergl.
Physiol._, 1913, iv.
[239] v. Frisch, K., “Der Farbensinn und Formensinn der Biene,” _Zoöl.
Jahrb. Abt. f. allg. Zoöl. u. Physiol._, 1914, xxxv. See also Ewald, W.
F., _Ztschr. f. Sinnesphysiol._, 1914, xlviii., 285.
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. Kellogg[241] 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.
[240] Loeb, J., _Der Heliotropismus der Tiere_, 1889.
[241] Kellogg, V. L., _Science_, 1903, xviii., 693.
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 temperature[243] 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]
[242] Loeb, J., _Arch. f. d. ges. Physiol._, 1906, cxv., 564.
[243] _Ibid._, 1893, liv., 81.
[244] Groom, Theo. T., and Loeb, J., _Biol. Centralbl._, 1890, x., 160;
Ewald, W. F., _Jour. Exper. Zoöl._, 1912, xiii., 591.
[245] Loeb, J., _Arch. f. d. ges. Physiol._, 1906, cxv., 564; Moore, A.
R., _Jour. Exper. Zoöl._, 1912, xiii., 573.
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 Cannon[246]
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.
[246] Cannon, W. B., _Bodily Changes in Pain, Hunger, Fear, and Rage_,
New York, 1915.
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]
[247] Setchell, W. A., _Science_, 1903, xxvii., 934.
[248] Duclaux, E., _Traité de microbiol._, 1898, i., 280.
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.
It took at 100° over sixteen hours
" " " 105-110° two to four hours
" " " 115° thirty to sixty minutes
" " " 125-130° five minutes or more
" " " 135° one to five minutes
" " " 140° one minute
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
Cullen[250] 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]
[249] A full discussion of the literature on temperature coefficients
is given in A. Kanitz’s book on _Temperatur and Lebensvorgänge_,
Berlin, 1915.
[250] Van Slyke, D. D., and Cullen, G. E., _Jour. Biol. Chem._, 1914,
xix., 141.
[251] These considerations may meet the objections of Krogh to the
application of the van’t Hoff rule of temperature effect on reaction
velocity to life phenomena. See also the discussion of this subject in
Kanitz’s book.
We will now show the rôle of the temperature coefficient upon phenomena
of development. F. R. Lillie and Knowlton[252] 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]
[252] Lillie, F. R., and Knowlton, E. P., _Zoöl. Bull._, 1897, i.
[253] Hertwig, O., _Arch. mikrosk. Anat._, 1898, li., 319. See also E.
Cohen, _Vorträge für Aerste über physikalische Chemie._ 2d ed. Leipzig,
1907.
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.
[254] Loeb, J., _Arch. f. d. ges. Physiol._, 1908, cxxiv., 411; Loeb
J., and Wasteneys, H., _Biochem. Ztschr._, 1911, xxxvi., 345; Loeb J.,
and Chamberlain, M. M., _Jour. Exper. Zoöl._, 1915, xix., 559.
TABLE X
INFLUENCE OF TEMPERATURE UPON THE TIME (IN MINUTES) REQUIRED FROM
INSEMINATION TO THE FIRST CELL DIVISION
-----------+-----------+----------------------------------
| _Arbacia_ |
Temperature+-----------+-------------+_Strongylocentrotus_
| Loeb and | Loeb and | _purpuratus_
| Wasteneys | Chamberlain |
| 1911 | 1915 |
-----------+-----------+-------------+--------------------
°C. | _Minutes_ | _Minutes_ | _Minutes_
3 | | | 532
4 | | | 469
5 | | | 352
6 | | | 275
7 | 498 | | 291
8 | 410 | 411 | 210
9 | 308 | 297.5 | 159
10 | 217 | 208 | 143
11 | 175 | 175 |
12 | 147 | 148 | 131
13 | | 129 |
14 | | 116 | 121
15 | 100 | 100 | 100
16 | 85.5 | |
17 | 70.5 | |
18 | 68 | 68 | 87
19 | | 65 | 78
20 | 56 | 56 | 75
21 | | 53.3 | 78
22 | 47 | 46 | 75
23 | | 45.5 | Upper temperature
24 | | 42 | limit
25 | 40 | 39.5 |
26 | 33.5 | |
27.5 | 34 | |
30 | 33 | |
31 | 37 | |
-----------+-----------+-------------+--------------------
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 Wasteneys[255] 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.
[255] _Loc. cit._
TABLE XI
TEMPERATURE COEFFICIENTS Q₁₀ FOR THE RATE OF SEGMENTATION AND
OXIDATIONS IN THE EGGS OF _Strongylocentrotus_ AND _Arbacia_
-----------+--------------------+-----------+----------------
| Q₁₀ for Rate of Segmentation in | Q₁₀ for Rate of
Temperature+--------------------+-----------+ Oxidations in
|_Strongylocentrotus_| _Arbacia_ | _Arbacia_
-----------+--------------------+-----------+----------------
°C. | | |
3-13 | 3.91 | | 2.18
4-14 | 3.88 | |
5-15 | 3.52 | | 2.16
7-17 | 3.27 | 7.3 | 2.00
8-18 | | 6.0 |
9-19 | 2.04 | 4.7 |
10-20 | 1.90 | 3.8 | 2.17
11-21 | | 3.3 |
12-22 | 1.74 | 3.1 |
13-23 | | 2.8 | 2.45
15-25 | | 2.5 | 2.24
16-26 | | 2.6 |
17.5-27.5 | | 2.2 | 2.00
20-30 | | 1.7 | 1.96
-----------+--------------------+-----------+----------------
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.
Kanitz[256] 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°.
[256] Kanitz, A., _loc. cit._, p. 123.
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.
[257] Loeb, J., and Chamberlain, M. M., _Jour. Exper. Zoöl._, 1915,
xix., 559.
TABLE XII
---------------+-------------
| _Latitude
_Temperature_ | of
| Variation_
---------------+-------------
°C. | _Minutes_
9 | 52.5
10 | 39.5
11 | 26.0
12 | 22.5
13 | 19.2
14 | 17.5
15 | 13.0
18 | 12.0
19 | 12.5
20 | 9.6
21 | 8.0
22 | 7.8
23 | 8.0
24 | 8.0
25 | 5.0
---------------+-------------
TABLE XIII
--------------+------------------------------
| TEMPERATURE COEFFICIENT OF
+-------------+----------------
_Temperature | _Latitude |
Interval_ | of | _Segmentation_
| Variation_ |
--------------+-------------+----------------
°C. | |
9-19 | 4.2 | 4.7
10-20 | 3.9 | 3.8
11-21 | 3.2 | 3.3
12-22 | 2.8 | 3.1
13-23 | 2.4 | 2.8
14-24 | 2.3 | 2.8
15-25 | 2.6 | 2.5
--------------+-------------+----------------
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.
[258] Loeb, J., and Ewald, W. F., _Biochem. Ztschr._, 1913, lviii., 179.
TABLE XIV
---------------+--------------------------------------------
_Temperature_ | _Time Required for Nineteen Heart-beats in
| the Embryo of Fundulus_
---------------+--------------------------------------------
°C. | _Seconds_
|
30 | 6.25
25 | 8.5
20 | 11.5
15 | 19.0
10 | 32.5
5 | 61.0
10 | 33.5
15 | 18.8
20 | 12.0
25 | 10.0
30 | 6.0
---------------+--------------------------------------------
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 seeds[259] 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_.
[259] Clausen, H., _Landwirtschaftl. Jahrb._, 1890, xix., 893.
[260] Matthaei, G. L. C., _Trans. Philosoph. Soc._, 1904, cxcvii., 47;
Blackman, F. F., _Ann. of Bot._, 1905, xix., 281.
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
------------+-------------+-------------+--------------+-------------+------------
0° to | 0° to | A | +35° to | +36° to | +42° to
-20°C. | +10°C. | (Normal | +37°C. | +41°C. | +46°C.
| | Forms) | | |
------------+-------------+-------------+--------------+-------------+------------
_ichnusoides|_polaris_ |_urticæ_ |_ichnusa_ |_polaris_ |_ichnusoides
(nigrita)_| | | | | (nigrita)_
_antigone |_fischeri_ |_io_ | ---- |_fischeri_ |_antigone
(iokaste)_| | | | | (iokaste)_
_testudo_ |_dixeyi_ |_polychloros_|_erythromelas_|_dixeyi_ |_testudo_
_hygiæa_ |_artemis_ |_antiopa_ |_epione_ |_artemis_ |_hygiæa_
_elymi_ |_wiskotti_ |_cardui_ | ---- |_wiskotti_ |_elymi_
_klymene_ |_merrifieldi_|_atalanta_ | ---- |_merrifieldi_|_klymene_
_weismanni_ |_porima_ |_prorsa_ | ---- |_porima_ |_weismanni_
------------+-------------+-------------+--------------+-------------+------------
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.
[261] Loeb, J., “The Poisonous Character of a Pure NaCl Solution.”
_Am. Jour. Physiol._, 1900, iii., 329; _Arch. f. d. ges. Physiol._,
1901, lxxxviii., 68; _Am. Jour. Physiol._, 1902, vi., 411; _Biochem.
Zischr._, 1906, ii., 81.
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
----------------+----------------------
_Concentration | _Cc. m/16 CaCl₂
of | Required for 50 c.c.
_NaCl_ | NaCl Solution_
----------------+----------------------
m. |
3/8 | 0.1
4/8 | 0.3
5/8 | 0.5
6/8 | 0.6
7/8 | 0.9
8/8 | 1.2-1.4
9/8 | 1.8-2.0
10/8 | 2.0-2.5
11/8 | 2.0?
12/8 | 3.0-3.5
13/8 | 6.0
----------------+----------------------
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.
[262] Loeb, J., _Jour. Biol. Chem._, 1915, xxiii., 423.
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
[263] Loeb, J., “On the Physiological Effects of the Valency and
Possibly the Electrical Charges of Ions,” _Am. Jour. Physiol._, 1902,
vi., 411.
[264] Loeb, J., _Jour. Biol. Chem._, 1914, xix., 431.
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 the | _In the Same Salts with the
Pure Salts_ | Addition of 1 c.c. m CaCl₂
| to 50 c.c. Solution_
-------------------+-----------------------------
LiCl about 6/32m | >5/8 m
NaCl m/2 | >14/8 m
KCl >11/16 m | >8/8 m
<6/8 m |
RbCl >8/8 m | >9/8 m
<7/8 m |
CsCl >3/8 m | >8/8 m
<4/8 m |
-------------------+-----------------------------
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. Robertson[266] 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. 3 m 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.
[265] Loeb, J., _Arch. f. d. ges. Physiol._, 1905, cvii., 252.
[266] Robertson, T. B., _Ergeb. d. Physiol._, 1910, x., 216.
[267] Loeb, J., _Biochem. Ztschr._, 1912, xlvii., 127.
The same experiments cannot be demonstrated in the sea-urchin egg,
first, because it can live neither in distilled water nor in very
dilute nor very concentrated solutions; and second, because it is not
separated as is the germ of the _Fundulus_ egg from the surrounding
solution by a membrane which is under proper conditions practically
impermeable for water and salts.
Nevertheless it can be shown that the results at which we arrived
in our experiments on _Fundulus_ are of a general application.
Osterhout[268] has shown that plants which grow in the soil or in
fresh water are readily killed by a pure NaCl solution of a certain
concentration, while they can resist the same concentration of NaCl if
some CaCl₂ is added. Wo. Ostwald[269] has shown the same for a species
of _Daphnia_. We, therefore, come to the conclusion that the injurious
action following an alteration in the constitution of the sea water
is in some of the cases due to an increase in the permeability of the
membranes of the cell, whereby substances can diffuse into the cell
which when the proper balance prevails cannot diffuse. For this balance
the ratio of the concentration of the salts with univalent cation Na
and K over those with bivalent cation Ca and Mg C_{Na+K salts}/C_{Ca+Mg
salts} is of the greatest importance.
[268] Osterhout, W. J. V., _Bot. Gazette_, 1906, xlii., 127; 1907,
xliv., 257; _Jour. Biol., Chem._, 1906, i., 363.
[269] Ostwald, Wo., _Arch. f. d. ges. Physiol._, 1905, cvi., 568.
6. The importance of this quotient appears in the so-called “behaviour”
of marine animals. We have mentioned the newly hatched larvæ of the
barnacle in connection with heliotropism. These larvæ swim in a trough
of normal sea water at the surface, being either strongly positively or
negatively heliotropic. They collect as a rule in two dense clusters,
one at the window and one at the room side of the dish. If such animals
are put into a solution of NaCl+KCl (in the proportion in which these
salts exist in the sea water), they will fall to the bottom unable
to rise to the surface. They will, however, rise to the surface and
swim energetically to or from the window if a certain quantity of any
of the chlorides of a bivalent metal, Mg, Ca, or Sr, is added, but
these movements will last only a few minutes when only one of these
three salts is added; and then the animals will fall to the bottom
again. If, however, two salts, _e. g._, MgCl₂ and CaCl₂, are added
the animals will stay permanently at the surface and react to light
as they would have done in normal sea water. These animals also can
resist comparatively large changes in the concentration of the sea
water, and it seemed of interest to find out whether the quotient
C_{NaCl+KCl}/C_{MgCl₂+CaCl₂}, which just allowed all the animals to
swim at the surface, had a constant value. The MgCl₂+CaCl₂ solution
was 3/8 m and contained the two metals in the proportion in which they
exist in the sea water; namely, 11.8 molecules MgCl₂ to 1.5 molecules
CaCl₂. The next table gives the result.[270] Since these experiments
lasted a day or more each, usually two different concentrations of
NaCl+KCl of the ratio 1:2 or 1:4 were compared in one experiment.
[270] Loeb, J., _Jour. Biol. Chem._, 1915, xxiii., 423.
TABLE XVIII
-----------+--------------+--------------+-----------
_Number |_Concentration| _C.c. 3/8 m |_Value of
of | of | CaCl₂+MgCl₂ | C_{Na+K}/
Experiment_| NaCl+KCl_ | Required_ | C_{Mg+Ca}_
-----------+--------------+--------------+-----------
1 | {m/16 | 0.3 | 27.8
| {m/8 | 0.4-0.5 | 37.0
| | |
2 | {m/8 | 0.5 | 33.3
| {m/4 | 0.9-1.0 | 35.1
| | |
3 | {3/16 m | 0.7 | 35.7
| {3/8 m | 1.3 | 38.5
| | |
4 | {m/8 | 0.5 | 36.0
| {m/2 | 1.8-1.9 | 39.2
| | |
5 | {m/4 | 0.8-0.9 | 39.2
| {m/2 | 1.6-1.7 | 40.3
| | |
6 | {5/16 m | 0.9 | 46.3
| {5/8 m | 1.7 | 49.0
| | |
7 | {3/16 m | 0.6 | 41.7
| {6/8 m | 2.4 | 41.7
-----------+--------------+--------------+-----------
These experiments indicate that the ratio of C_{Na+K}/C_{Ca+Mg} remains
very nearly constant with varying concentrations of C_{Na+K}.
In former experiments on jellyfish the writer had shown that there
exists an antagonism between Mg and Ca[271], and this observation
was subsequently confirmed by Meltzer and Auer[272] for mammals. It
was observed that in a solution of NaCl+KCl+MgCl₂ the larvæ of the
barnacle were also not able to remain at the surface for more than a
few minutes, while an addition of some CaCl₂ made them swim permanently
at the surface. Various quantities of MgCl₂ were added to a mixture of
m/4 or m/2 NaCl+KCl, to find out how much CaCl₂, was required to allow
them to swim permanently at the surface.
[271] Loeb, J., _Jour. Biol. Chem._, 1905-06, i., 427.
[272] Meltzer, S. J., and Auer, J., _Am. Jour. Physiol._, 1908, xxi.,
400.
TABLE XIX
--------------------------------------+----------------------------
|_C.c. of m/16 CaCl₂ Necessary
| to Induce the Majority of
| the Larvæ to Swim in_
+--------------+-------------
| m/2 (Na+K) | m/4 (Na+K)
--------------------------------------+--------------+-------------
50 c.c. NaCl+KCl+0.75 c.c. 3/8 m MgCl₂| | 0.2
50 c.c. NaCl+KCl+ 1.5 c.c. 3/8 m MgCl₂| 0.4 | 0.3
50 c.c. NaCl+KCl+ 2.5 c.c. 3/8 m MgCl₂| 0.4 | 0.4
50 c.c. NaCl+KCl+ 5.0 c.c. 3/8 m MgCl₂| 0.7-0.8 | 0.7-0.8
50 c.c. NaCl+KCl+10.0 c.c. 3/8 m MgCl₂| 1.6 | 1.6
50 c.c. NaCl+KCl+15.0 c.c. 3/8 m MgCl₂| 1.8 |
50 c.c. NaCl+KCl+20.0 c.c. 3/8 m MgCl₂| 1.8 |
--------------------------------------+--------------+-------------
In order to interpret these figures correctly we must remember that
we are dealing with two different antagonisms, one between the salts
with univalent and bivalent metals and the other between Mg and Ca.
The former antagonism is satisfied by the addition of Mg, inasmuch
as enough Mg was present for this purpose in all solutions. What was
lacking was the balance between Mg and Ca. The experiments in Table XIX
therefore answer the question of the ratio between Mg and Ca. If we
consider only the concentrations of Mg between 2.5 and 10.0 c.c. 3/8 m
MgCl₂--which are those closest to the normal concentration of Mg in the
sea water--we notice that C_{Ca} must vary in proportion to C_{Mg}. If
we now combine the results of this and the previous paragraph we may
express them in the form of the _theory of physiologically balanced
salt solutions, by which we mean that in the ocean (and in the blood or
lymph) the salts exist in such ratio that they mutually antagonize the
injurious action which one or several of them would have if they were
alone in solution._[273] This law of physiologically balanced solutions
seems to be the general expression of the effect of changes in the
constitution of the salt solutions for marine or all aquatic organisms.
[273] This theory was first expressed by the writer in _Am. Jour.
Physiol._, 1900, iii., 434.
This chapter would not be complete without an intimation of the rôle of
buffers in the sea water and the blood, by which the reaction of these
media is prevented from changing in a way injurious to the organism.
These buffers are the carbonates and phosphates. Instead of saying that
the organisms are adapted to the medium, L. Henderson has pointed out
the fitness of the environment for the development of organisms and one
of these elements of fitness are the buffers against alterations of
the hydrogen ion concentration.[274] The ratio in which the salts of
the different metals exist in the sea water is another. It is obvious
that the quantitative laws prevailing in the effect of environment
upon organisms leave no more room for the interference of a “directing
force” of the vitalist than do the laws of the motion of the solar
system.
[274] Henderson, L., _The Fitness of the Environment_. See also
Michaelis, L., _Die Wasserstoffionenconzentration_. Berlin. 1914.
CHAPTER XII
ADAPTATION TO ENVIRONMENT
1. It is assumed by certain biologists that the environment influences
the organism in such a way as to increase its adaptation. Were this
correct it would not contradict a purely physicochemical conception of
life; it would only call for an explanation of the mechanism by which
the adaptation is brought about. There are striking cases on record
which warn us against the universal correctness of the view that the
environment causes an adaptive modification of the organism. Thus
the writer pointed out in 1889 that positive heliotropism occurs in
organisms which have no opportunity to make use of it,[275] _e. g._,
_Cuma rathkii_, a crustacean living in the mud, and the caterpillars
of the willow borer living under the bark of the trees. We understand
today why this should be so, since heliotropism depends upon the
presence of photosensitive substances, and it can readily be seen that
the question of use or disuse has nothing to do with the production of
certain harmless chemical compounds in the body. A much more striking
example is offered in the case of galvanotropism. Many organisms
show the phenomenon of galvanotropism, yet, as the writer pointed
out years ago, galvanotropism is purely a laboratory product and no
animal has ever had a chance or will ever have a chance to be exposed
to a constant current except in the laboratory of a scientist. This
fact is as much of a puzzle to the selectionist and to the Lamarckian
(who would be at a loss to explain how outside conditions could have
developed this tropism) as to the vitalist who would have to admit
that the genes and supergenes indulge occasionally in queer freaks
and lapses. The only consistent attitude is that of the physicist who
assumes that the reactions and structures of animals are consequences
of the chemical and physical forces, which no more serve a purpose than
those forces responsible for the solar systems. From this viewpoint it
is comprehensible why utterly useless tropisms or structures should
occur in organisms.
[275] Loeb, J., _Der Heliotropismus der Tiere and seine Übereinstimmung
mit dem Heliotropismus der Pflanzen_. Würzburg, 1890 (appeared in 1889).
2. A famous case for the apparent adaptation of animals to environment
has been the blind cave animals. It is known that in caves blind
salamanders, blind fishes, and blind insects are common, while such
forms are comparatively rare in the open. This fact has suggested the
idea that the darkness of the cave was the cause of the degeneration
of the eyes. A closer investigation leads, however, to a different
explanation. Eigenmann has shown that of the species of salamanders
living habitually in North American caves, two have apparently
quite normal eyes. They are _Spelerpes maculicauda_ and _Spelerpes
stejnegeri_. Two others living in caves have quite degenerate eyes,
_Typhlotriton spelæus_ and _Typhlomolge rathbuni_. If disuse is the
direct cause of blindness we must inquire why _Spelerpes_ is not blind.
Another difficulty arises from the fact that a blind fish
_Typhlogobius_ is found in the open (on the coast of southern
California) in shallow water, where it lives under rocks in holes
occupied by shrimps. The question must again be raised: How can it
happen that in spite of exposure to light _Typhlogobius_ is blind?
The most important fact is perhaps the one found by Eigenmann in the
fishes of the family of Amblyopsidæ. Six species of this group live
permanently in caves, are not found in the open, and have abnormal
eyes, while one lives permanently in the open, is never found in caves,
and one comes from subterranean springs. The one form which is found
only in the open, _Chologaster cornutus_, has a simplified retina as
well as a comparatively small eye, in other words, its eye is not
normal. This indicates the possibility that the other representatives
which are found only in caves also might have abnormal eyes even if
they had never lived in caves.
Through these facts the old idea becomes questionable, namely, that the
cave animals had originally been animals with normal eyes which owing
to disuse had undergone a gradual hereditary degeneration.
Recent experiments made on the embryos of the fish _Fundulus_ have
yielded the result that it is possible to produce blindness in fish
by various means other than lack of light.[276] Thus the writer found
that by crossing the egg of _Fundulus_ with the sperm of a widely
different species, namely, _Menidia_, blind embryos were produced
very frequently; that is to say such embryos had the degenerate eyes
characteristic of blind cave fishes. Very often no other external
trace of an eye, except a gathering of pigment, could be found, while
a close histological examination would possibly have resulted in the
demonstration of rudiments of a lens and other tissues of the eye.
[276] Loeb, J., _Biol. Bull._, 1915, xxix., 50.
Another method of producing blind fish embryos consists in exposing the
egg immediately, or soon after fertilization, to a temperature between
0° and 2° C. for a number of hours. Many embryos are killed by this
treatment, but those which survive behave very much like the hybrids
between _Fundulus_ and _Menidia_, _i. e._, a number of them have quite
degenerated eyes. If the eggs have once formed an embryo they can be
kept at the temperature of 0° for a month or more without giving rise
to blind animals. Occasionally such rudimentary eyes were also observed
when eggs were kept in a solution containing a trace of KCN. Stockard
has succeeded in producing cyclopean eyes in _Fundulus_ by adding an
excess of magnesium salt to the sea water in which the eggs developed
or by adding alcohol, and McClendon has confirmed and added to these
results.
The writer tried repeatedly, but in vain, to produce _Fundulus_ with
deficient eyes by keeping the embryos in the dark. Sperm and egg
were not allowed to be exposed to the light yet the embryos without
exception had normal eyes.
F. Payne raised sixty-nine successive generations of a fly _Drosophila_
in the dark, but the eyes and the reaction of the insects to light
remained perfectly normal.
Uhlenhuth has recently demonstrated in a very striking way that the
development of the eyes does not depend upon the influence of light
or upon the eyes functioning. He transplanted the eyes of young
salamanders into different parts of their bodies where they were no
longer connected with the optic nerves. The eyes after transplantation
underwent a degeneration which was followed by a complete regeneration.
He showed that this regeneration took place in complete darkness and
that the transplanted eyes remained normal in salamanders kept in
the dark for fifteen months. Hence the eyes which were no longer in
connection with the central nervous system, which had received no
light, and could not have functioned, regenerated and remained normal.
The degeneration which took place in the eyes immediately after being
transplanted was apparently due to the interruption of the circulation
in the eye, and the regeneration commenced in all probability with the
re-establishment of the circulation in the transplanted organ.
In our own experiments it can be shown that the circulation in the
embryo was deficient in all cases where the eyes degenerated. The
hybrids between _Fundulus_ and _Menidia_ have often a beating heart
but rarely a circulation (although they form blood); and the same
phenomenon occurred in the embryos which were exposed to a low
temperature at an early period of their lives. Hence all the facts
agree that conditions which lead to an abnormal circulation (and
consequently also to an abnormal or inadequate nutrition of the
embryonic eye) may prevent development and lead to the formation of
blind fishes. Eigenmann states that no blood-vessels enter the eye of
the blind cave salamander _Typhlotriton_. The presence or absence of
light does not usually interfere with the circulation or nutrition of
the embryonic eye, and hence does not as a rule lead to the formation
of degenerated eyes.
This would lead us to the assumption that the blind fish owe their
deficiency not to lack of light but to a condition which interferes
with the circulation in the embryonic eye. Such a condition might be
brought about by an anomaly in the germ plasm or in one chromosome,
the nature and cause of which we are not able to determine at present;
but which, since it occurs in the germ plasm or the chromosomes, must
be hereditary. This would explain why it is, that animals with perfect
eyes may occur in caves and why perfectly blind animals may occur
in the open. It leaves, however, one point unexplained; namely, the
greater frequency of blind species in caves or in the dark and the
relative scarcity of such forms in the open.
Eigenmann has shown that all those forms which live in caves were
adapted to life in the dark before they entered the cave.[277] These
animals are all negatively heliotropic and positively stereotropic, and
with these tropisms they would be forced to enter a cave whenever they
are put at the entrance. Even those among the Amblyopsidæ which live
in the open have the tropisms of the cave dweller. This eliminates the
idea that the cave adapted the animals for the life in the dark.
[277] Cuénot has proposed the term preadaptation for such cases and
this term expresses the situation correctly. Cuénot, L., _La Génèse des
Espèces animales_. Paris, 1911.
Only those animals can thrive in caves which for their feeding and
mating do not depend upon visual mechanisms; and conversely, animals
which are not provided with visual mechanisms can hold their own in the
open, where they meet the competition of animals which can see, only
under exceptional conditions. This seems to account for the fact that
in caves blind species are comparatively more prevalent than in the
open.
In other words, the adaptation of blind animals to the cave is only
apparent; they were adapted to cave life before they entered the cave.
Many animals are obviously burdened with a germinal abnormality giving
rise to imperfection and smallness of the eye--the hereditary factor
involved may have to do with the development of the blood-vessels and
lymphatics of the eye. Such mutants can survive more easily in the
cave, where they do not have to meet the competition of seeing forms,
than in the open. In man also an hereditary form of blindness is known,
the so-called hereditary glaucoma. It has nothing to do with light, but
the disease seems to be due to an hereditary anomaly of the circulation
in the eye.
Kammerer[278] has recently reported that by keeping the blind European
cave salamander _Proteus anguinus_ under certain conditions of
illumination he succeeded in producing two specimens with larger eyes.
According to him the eyes of _Proteus_ may develop to a certain point
and then retrogress again. He states that by keeping young salamanders
alternately for a week or two in sunlight and in a dark room where
they were exposed to red incandescent light, two males formed somewhat
larger eyes. The first year no alteration was visible. In the second
year a slight increase in the size of the eyes was noticeable under the
skin. In the third year the eye protruded slightly and this increased
somewhat in the fourth year.
[278] Kammerer, P., _Arch. f. Entwcklngsmech._, 1912, xxxiii., 349.
There is thus far only one case on record in animal biology in which
the light influences the formation of organs. The writer found that
the regeneration of the polyps of the hydroid _Eudendrium_ does not
take place if the animals are kept in the dark, while the polyps will
regenerate if exposed to the light;[279] and the time of exposure
may be rather short according to Goldfarb.[280] It is possible that
_Proteus_ resembles in this respect _Eudendrium_; it should be stated,
however, that of many different forms tried by the writer over a number
of years, _Eudendrium_ was the only one which gave evidence of such
an influence of light. Of course it is not impossible that the light
might influence reflexly the development of blood-vessels in the eyes
of certain animals, _e. g._, _Proteus_, and thus allow the eyes of
_Proteus_ to grow a little larger.
[279] Loeb, J., _Arch. d. f. ges. Physiol._, 1896, lxiii., 273.
[280] Goldfarb, A. J., _Jour. Exper. Zoöl._, 1906, iii., 129; 1910,
viii., 133.
We therefore come to the conclusion that it is not the cave that made
animals blind but that animals with a hereditary tendency towards a
degeneration of the eyes can survive in a cave while they can only
exceptionally survive in the open. The cause of the degeneration is a
disturbance in the circulation and nutrition of the eye, which is as a
rule independent of the presence or absence of light.
We may by way of a digression stop for a moment to consider the most
astonishing and uncanny case of adaptation; namely, the formation of
the transparent refractive media, especially the lens in front of the
retina. It is due to these media that the rays which are sent out
by a luminous point can be united to an image point on the retina.
One part of this process is understood; namely, the formation of a
lens. Wherever the optic cup of the embryo is transplanted under the
epithelium the latter will be transformed into a transparent lens.
When the upper edge of the iris is injured in the salamander so that
the cells can multiply, the mass of newly formed cells also becomes
transparent and a lens is formed. This indicates the existence
of a substance in the optic cup which makes the epithelial cells
transparent; and which also limits the size of the lens which is
formed. The lens is not always a perfect optical instrument, on the
contrary, it is as a rule somewhat defective. Of course, a great many
details concerning the process of lens regeneration have still to be
worked out.
3. It is well known that most marine animals die if put into
fresh water and _vice versa_; and in salt lakes or ponds with a
concentration of salt so high that most marine animals would succumb
if suddenly transferred to such a solution we have a limited fauna
and flora. The common idea is that marine animals become adapted to
fresh water or _vice versa_; or to the conditions in salt lakes;
especially if the changes take place gradually. Yet it can be shown
that the existence of these different faunas can be explained without
the assumption of an adaptive effect of the environment. The writer
has worked with a marine fish _Fundulus_ whose eggs develop naturally
in sea water which, however, will develop just as well in distilled
water; and the young fish hatching in distilled water live and grow in
this medium. Most of the adult fish die after several days, when put
suddenly into distilled water, but they can live in fresh water which
contains only a trace of salt. They can also live in very concentrated
sea water, _e. g._, twice the normal concentration. Suppose that a bay
of the ocean containing such fish should suddenly become landlocked
and the concentration of the sea water be thus raised to twice its
natural amount; the majority of forms would die and only _Fundulus_
and possibly a few other species with the same degree of resistance
would survive. An investigator examining the salinity of the water
and not knowing the natural resistance of _Fundulus_ to changes in
concentration would be inclined to assume that he had before him an
instance of a gradual adaptation of the fish to a higher concentration
of the sea water; whereas the fish was already immune to this high
concentration before coming in contact with it.
This fish seemed a favourable object from which to find out how far
an adaptation to the environment really existed; and the result was
surprising. By changing the concentration of the sea water gradually
it is possible to raise the natural resistance of the fish only a
trifle, not much over ten per cent. The concentration of the natural
sea water is a little over that of a m/2 solution of NaCl+KCl+CaCl₂ in
the proportion in which these three salts exist in the sea water. When
adult _Fundulus_ are put into a 10/8 m solution of NaCl+KCl+CaCl₂ in
the proportion in which these salts occur in sea water they die in less
than a day, but when put from sea water directly into a 8/8 m or 9/8
m solution they can live indefinitely. It was found[281] that if the
concentration of the sea water was raised gradually (by m/8 a day) the
fish on the fifth day could resist a 10/8 m solution of NaCl+KCl+CaCl₂
for a month (or possibly indefinitely; the experiment was discontinued
after that period). When a 10/8 m solution was allowed to become more
concentrated slowly by evaporation (at room temperature) all the fish
died rapidly when the concentration was 12/8 m or even below. In higher
concentrations they can live only a day or two. These experiments show
that while the fish is naturally immune to a 9/8 m NaCl+KCl+CaCl₂
solution, by the method of slowly raising the concentration it may be
made to tolerate a 10/8 m or 11/8 m solution, but not more. These fish
when once adapted to a 10/8 m solution can be put suddenly into a very
weak solution, _e. g._, a m/80 NaCl, without suffering and when brought
back into a 10/8 m solution of NaCl+KCl+CaCl₂ they will continue to
live. If they remain for several days in the weak solution their power
of resistance to 10/8 m NaCl+KCl+CaCl₂ solution is weakened.
[281] Loeb, J., _Biochem. Ztschr._, 1913, liii., 391.
What change takes place when the fish is made more resistant and why
is its normal resistance so great? The answer based on the writer’s
experiments seems to be as follows: _Fundulus_ is comparatively
resistant to sudden changes in the concentration of the sea water
between m/80 and 9/8 m because it possesses a comparatively
impermeable skin whose permeability is not seriously altered by
sudden changes within these limits of concentration; while if these
limits are exceeded and the fish are brought suddenly into too high
a concentration the skin becomes permeable and the fish dies, the
gills becoming unfit for use or nerves being injured by the salt which
diffuses into the fish.
The fact, that by slowly raising the concentration to 10/8 m the fish
may resist this limit, is in reality no adaptation. There is no sharp
limit between the injurious and non-injurious concentration. We have
seen that the fish is naturally immune to a 9/8 m solution. It is also
naturally immune to a 10/8 m or 11/8 m solution if we give it time to
compensate the injurious effects of a 10/8 m solution by the repairing
action of its blood or kidneys. Beyond this no rise is possible. In
reality adaptation does not exist in this case.
In former experiments the writer had shown that a pure NaCl solution
of that concentration in which this fish naturally lives kills it
very rapidly, while it lives in such a solution indefinitely if a
little CaCl₂ is added. The explanation of this fact is that the pure
NaCl solution is able to diffuse into the tissues of the animal while
the addition of a trace of CaCl₂ renders the membrane practically
impermeable to NaCl. The question then arose whether it was possible to
make the fish more resistant to a pure NaCl solution of sufficiently
high concentration and how this could be done. On the basis of the
idea of an adaptive effect of the environment we should expect that by
gradually raising the concentration of a pure NaCl solution the latter
would gradually alter the animal and make it more resistant. The method
of procedure suggested was therefore to put the fish first in low and
gradually into increasing concentrations of NaCl. This method was tried
and found futile for the purpose. _Fundulus_ when put from sea water
(after having been washed) into a 6/8 m NaCl solution die in about
four hours. When kept previously in a weaker NaCl solution they die if
anything more quickly. But it is possible to make them live longer
in a 6/8 m solution of NaCl; we have to proceed, however, by a method
which is in contrast with the ideas of the adaptive influence of the
environment. When the fish are first treated with sea water (or with
a mixture of NaCl+KCl+CaCl₂) of a higher concentration so that they
become adapted to a 10/8 m solution of NaCl+KCl+CaCl₂ or to 10/8 m sea
water, they become also more resistant to an otherwise toxic solution
of NaCl. Fish taken directly from sea water were killed in less than
four hours when put into a 6/8 m NaCl solution, while fish of the same
lot previously adapted to 10/8 m sea water in the manner described
above lived two or three days in a 6/8 m NaCl solution.[282]
[282] Loeb, J., _Biochem. Ztschr._, 1913, liii., 391.
It is not impossible that it was the high concentration of calcium
in the 10/8 m sea water which rendered the fish more immune to a
subsequent treatment with NaCl. We know why a pure NaCl solution kills
them and we also know why the addition of CaCl₂ protects them against
this pernicious effect. It is rather strange that where the conditions
of the experiments are clear we find nothing to indicate an adaptive
effect of the environment.
4. Ehrlich’s work on trypanosomes seems to indicate a remarkable
power of adaptation on the part of organisms to certain poisons. If
the writer understands these experiments correctly they consisted
in infecting a mouse with a certain strain of trypanosomes, and
treating it with a certain arsenic compound, which inhibited somewhat
the propagation of the parasites but did not kill them all. Four or
five days later trypanosomes from this mouse were transmitted to
another mouse and after twenty-four hours this mouse was treated with
a stronger dose of the same arsenic compound; and this process was
repeated. After the third transmission or later, the trypanosomes can
resist considerably higher doses of the same poison than at first and
this resistance is retained for years. Ehrlich seems to have taken
it for granted that he had succeeded in transforming the surviving
trypanosomes into a type which is permanently more resistant to the
arsenic compound than was the original strain.
The writer is not entirely convinced that in these experiments
a possibility was sufficiently considered which is suggested by
Johannsen’s experiments on the importance of pure lines in work on
heredity. According to this author a strain of trypanosomes taken at
random should, in all likelihood, contain a population consisting of
strains with different degrees of resistance. If a high but not the
maximal concentration of an arsenic compound is repeatedly injected
into the infected mice the weaker populations of trypanosomes are
killed and only the more resistant survive. These of course continue to
retain their resistance if transplanted to hosts of the same species.
According to this interpretation the arsenic-fast strain may possibly
have existed before the experiments were made, and Ehrlich’s treatment
consisted only in eliminating the less resistant strains.
On the other hand, it has been shown that if an arsenic-fast strain
of trypanosomes is carried through a tsetse fly it loses its
arsenic-fastness. This fact may possibly eliminate the applicability
of the pure line theory to a discussion of the nature of the
arsenic-fastness, but it seems that further experiments are desirable.
5. Dallinger stated that he succeeded in adapting certain protozoans to
a temperature of 70° C. by gradually raising their temperature during
several years. It is desirable that this statement be verified; until
this is done doubts are justified. Schottelius found that colonies
of _Micrococcus prodigiosus_ when transferred from a temperature of
22° to that of 38° no longer formed pigment and trimethylamine. After
the cocci had been cultivated for ten or fifteen generations at 38°
they failed to form pigment even when transferred back to 22° C.
Dieudonné[283] used _Bacillus fluorescens_ for similar purposes. At
22° it forms a fluorescing pigment and trimethylamine, but not at 35°.
By constantly cultivating this bacillus at 35° Dieudonné found that
after the fifteenth generation had been cultivated at 35° the bacillus
produced pigment and trimethylamine at 35°. Davenport and Castle[284]
found that tadpoles of a frog kept at 15° went into heat rigour at 40.3°
C., while those kept for twenty-eight days at 25° were not affected by
this temperature but went into heat rigour at 43.5°. When the latter
tadpoles were put back for seventeen days to a temperature of 15°
they had lost their resistance to high temperature partially, but not
completely, since they went into heat rigour at 41.6°. The authors
suggest that this adaptation to a higher temperature is due to a loss
of water on the part of protoplasm, whereby the latter becomes more
resistant to an increase in temperature. This idea was put to a test by
Kryž[285], who found that the coagulation temperature of their muscle
plasm is not altered by keeping cold-blooded animals at different
temperatures.
[283] Dieudonné, A., _Arb. a. d. kais. Gesndhtsmt._, 1894, ix., 492.
[284] Davenport, C. B., and Castle, W. E., _Arch. f. Entwcklngsmech._,
1896, ii., 227.
[285] Kryž, F., _Arch. f. Entwcklngsmech._, 1907, xxiii., 560.
Loeb and Wasteneys[286] found that _Fundulus_ taken from a low
temperature of 10° C. die in less than two hours when suddenly
transferred to sea water of 29° C.; and in a few minutes if suddenly
transferred to a temperature of 35° C. If, however, the fish were
transferred to a temperature of 27° C. for forty hours they could
live indefinitely in sea water of 35°. By exposing the fish each day
two hours to a gradually rising temperature they could render them
resistant to a temperature of 39°. The remarkable fact was that fish
if once made resistant to a high temperature (35°) did not lose this
resistance when kept for four weeks at from 10° to 14° C. Control fish
taken from the same temperature died in from two to four minutes;
immunized fish taken from 10° and put directly to 35° C. lived for many
hours or indefinitely. They will even retain this immunity when kept
for two weeks at a temperature of 0.4° C.
[286] Loeb, J., and Wasteneys, H., _Jour. Exper. Zoöl._, 1912, xii.,
543.
Why is it that an animal can in general resist a high temperature
better if the latter is raised gradually than when it is raised
suddenly? Physics offers us an analogy to this phenomenon in the
experience that glass vessels which burst easily when their temperature
is raised suddenly, remain intact when the temperature is raised
gradually. Glass is a poor conductor of heat and when the temperature
is raised suddenly inside a glass cylinder the inner layer of the
cylinder expands while the outer layer on account of the slowness of
conduction of heat does not expand equally and the cylinder may burst.
We might assume that the sudden increase in temperature brings about
certain changes in the cells (_e. g._, an increase in permeability or
destruction of the surface layer?). If the rise of temperature occurs
gradually the blood or lymph or the cell sap may have time to repair
the damage, and this repair seems to be irreversible, at least for
some time, as the experiments on _Fundulus_ seem to indicate. If the
temperature rises too rapidly the damage cannot be repaired quickly
enough by the cell or body liquids.
It is also to be considered that substances might be formed in the body
at a higher temperature which do not exist at a lower temperature, and
_vice versa_, and this might explain results like those of Schottelius
or Dieudonné and many others.
6. The theory of an adapting effect of the environment has often been
linked with the assumption of the inheritance of acquired characters.
The older claims of the hereditary transmission of acquired characters,
such as Brown-Séquard’s epilepsy in guinea pigs after the cutting of
the sciatic nerve, have been shown to be unjustified or have found
a different and more rational explanation. Recently P. Kammerer has
claimed to have proven by new experiments that by environmental
changes, hereditary changes can be produced.
It has been mentioned already that the mature male frogs and toads
possess during the breeding season lumps on the thumbs or arms which
are pigmented and which bear numerous minute horny black spines; these
secondary sexual characters serve the male frog in holding the females
in the water during copulation. There is one species which does not
possess this sexual character, namely the male of the so-called midwife
toad (_Alytes obstetricans_). In this species the animals copulate on
land, and it is natural to connect the lack of this secondary sexual
character in the male with its different breeding habit. Kammerer
now forced such toads to copulate in water instead of on land (by
keeping the animals in a terrarium with a high temperature). He makes
the statement that by forcing the parents to lay their eggs during
successive spawning periods in water he finally obtained offspring
which under normal temperature conditions lay their eggs naturally
in water; in other words, they have changed their habits. We will
not discuss this part of his statement since the breeding habits of
animals in captivity are liable to be abnormal. But Kammerer makes
the further important statement[287] that the male offspring of such
couples will in the third generation produce the swelling on the thumb
and the usual roughness, and in the fourth generation black pads and
hypertrophy of the muscles of the forearm will appear. In other words,
he reports having succeeded in producing an inheritance of an acquired
morphological character which has never been known to occur in this
species. Bateson, on account of the importance of the case, wished to
examine it more closely and I will quote his report.
The systematists who have made a special study of _Batrachia_
appear to be agreed that _Alytes_ in nature does not have these
structures; and when individuals possessing them can be produced
for inspection it will, I think, be time to examine the evidence
for the inheritance of acquired characters more seriously. I wrote
to Dr. Kammerer in July, 1910, asking him for the loan of such
a specimen and on visiting the Biologische Versuchsanstalt in
September of the same year I made the same request, but hitherto
none has been produced. In matters of this kind much generally
depends on interpretations made at the time of observation; here,
however, is an example which could readily be attested by preserved
material.[288]
[287] Kammerer, P., _Arch. f. Entwcklngsmech._, 1909, xxviii., 448.
[288] Bateson, W., _Problems of Genetics_, pp. 201-202. Yale University
Press, 1913.
More recently the same author has reported another hereditary
morphological change brought about by outside conditions.[289] A
certain salamander (_Salamandra maculosa_) has yellow spots on a
generally dark skin. Kammerer states that if such salamanders are
kept on a yellow ground they become more yellow, not by an extension
of the chromatophores (which would not be surprising) but by actual
multiplication and growth of the yellow pigment cells; while the
black skin is inhibited in its growth. The reverse is true if these
salamanders are kept on black soil; in this case according to Kammerer
the growth of the yellow cells of the skin is inhibited while the black
part of the skin grows. Curiously enough, according to him, these
induced changes are hereditary. Here again we are dealing with the
inheritance of an acquired morphological character.
[289] Kammerer, P., _Arch. f. Entwcklngsmech._, 1913, xxxvi., 4.
Megusar[290] has repeated Kammerer’s experiments on salamanders but
contradicts him by stating that the colour of the soil has no influence
on the colouration of salamanders. Of course, we know the phenomenon
of colour adaptation in which the animal changes its colour pattern
according to the environment. This is an effect of the retina image on
the skin and has been interpreted by the writer as a case of colour
telephotography, for which no physical explanation has yet been
found.[291] This phenomenon, however, does not lead to any hereditary
change of colour.
[290] Werner, F., _Biol. Centralbl._, 1915, xxxv., 176.
[291] Loeb, J., _The Mechanistic Conception of Life_. Chicago, 1912.
Kammerer makes many statements on the heredity of acquired
modifications of instinct; indeed he claims that an interest in music
on the part of parents produces offspring with musical talent. In such
claims much depends upon the subjective interpretation of the observer.
The writer is not aware that there is at present on record a single
adequate proof of the heredity of an acquired character. We have
records of changes in the offspring by poisoning the germ plasm by
alcohol given to parents--as in Stockard’s well-known experiments--or
by exposing butterflies to extreme temperatures, but in these cases
the germ cells were poisoned or altered by the alcohol or by chemical
compounds produced at very low or very high temperatures. This is of
course an entirely different thing from stating that by inducing the
midwife toad to lay its eggs in the water the male offspring acquires
the pads and horns of other species of frogs on its thumb; or that by
keeping black salamanders on yellow paper the offspring is more yellow.
Yet if there is an inheritance of acquired characters which can in
any way throw light on the so-called phenomena of adaptation it must
consist in results such as Kammerer claims to have obtained.
While the writer does not decline to accept Ehrlich’s interpretation
of the arsenic-fast strains of trypanosomes or Kammerer’s statements
in regard to the inheritance of acquired character, he feels that more
work should be done before they can be used for our problem.
7. This attitude leaves us in a quandary. The whole animated world is
seemingly a symphony of adaptation. We have mentioned already the eye
with its refractive media so well curved and placed that a more or
less perfect image of the outside objects is focussed exactly on the
retina; and this in spite of the fact that lens and retina develop
independently; we have mentioned and discussed the cases of instincts
or automatic arrangements which are required to perpetuate life--the
attraction of the two sexes and the automatic mechanisms by which sperm
and egg are brought together; the maternal instincts by which the
young are taken care of; and all those adaptations by which animals
get their food and the suitable conditions of preservation. Can we
understand all these adaptations, without a belief in the heredity
of acquired characters? As a matter of fact the tenacity with which
some authors cling to such a belief is dictated by the idea that
this is the only alternative to the supra-naturalistic or vitalistic
ideas. The writer is of the opinion that we do not need to depend
upon the assumption of the heredity of acquired characters, but that
physiological chemistry is adequate for this purpose.
The earlier writers explained the growth of the legs in the tadpole of
the frog or toad as a case of an adaptation to life on land. We know
through Gudernatsch that the growth of the legs can be produced at
any time even in the youngest tadpole, which is unable to live on the
land, by feeding the animal with the thyroid gland. As we have stated
in Chapter VII, it is quite possible that in nature the legs of the
tadpole begin to grow when enough of the thyroid or a similar compound
has been formed or is circulating in the animal.
It might justly be claimed as a case of adaptation that the egg
attaches itself to the wall of the uterus and calls forth the formation
of the decidua. We have mentioned the observation of Leo Loeb that the
corpus luteum of the ovary gives off a substance to the blood which
alters the tissues in the uterus in such a way that contact with any
foreign body (_e. g._, the egg) induces this decidua formation. Again
what appeared as adaptation when unknown turns out to be a result of
the action of a definite chemical substance circulating in the body.
It appears as a case of adaptation that the eggs of the majority of
animals cannot develop without a spermatozoön, and yet we can imitate
the activating effect of a spermatozoön on the egg by definite chemical
compounds, which leads to the suggestion that the activating effect of
the spermatozoön on the egg might be due to the fact that it carries
such a compound.
The wonderful adaptations exhibited in the mating instincts seem to be
due to definite substances secreted by the sex glands, as was shown
by Steinach (Chapter VII). Here, again, the process as popularly
conceived, is the reverse of the truth; those survive that have the
equipment,--they did not acquire the equipment under the influence of
environment.
It is absolutely imperative for green plants that their stems and
leaves be exposed to the light since only in this way are they able
to form carbohydrates; and it is equally essential that the roots
should grow into the soil so that the plant may get the nitrates and
phosphates required to build up its proteins and nucleins. This result
is, in the language of adaptationists, brought about by an adaptive
response of the plant to the light. In reality this adaptive response
is due (Chapter X) to the presence of a photosensitive substance
present in almost all green plants.
Lewis has shown that if the optic cup is transplanted under the skin of
a young larva into any part of the body the skin in contact with the
optic cup will form a lens; it looks as if a chemical substance from
the optic cup were responsible for the formation of the lens.
These examples might be multiplied indefinitely. They all indicate that
apparent morphological and instinctive adaptations are merely caused
by chemical substances formed in the organism and that there is no
reason for postulating the inheritance of acquired characters. We must
not forget that there are just as many cases where chemical substances
circulating in the body lead to indifferent or harmful results. As an
example of the first type, we may mention the existence of heliotropism
in animals living in the dark, of the latter type, the inheritance of
deficiencies like colour-blindness or glaucoma.
While it is possible for forms with moderate disharmonies to survive,
those with gross disharmonies cannot exist and we are not reminded
of their possible existence. As a consequence the cases of apparent
adaptation prevail in nature.
The following observation may serve to give an idea how small is
the number of existing or durable forms compared with the number of
forms incapable of existence. We have mentioned the fact observed by
Moenkhouse, the writer, and Newman, that it is possible to fertilize
the eggs of each marine bony fish with the sperm of practically every
other marine bony fish. The number of teleosts at present in existence
is about ten thousand. If we accomplish all possible hybridizations,
one hundred million different crosses will result. Of these only a
small fraction of one per cent. can live (see Chapter I), and it is
generally the lack of a proper circulation which inhibits them from
reaching maturity. It is, therefore, no exaggeration to state that the
number of species existing today is only an extremely small fraction of
those which can and possibly do originate, but which escape our notice
and disappear because they cannot live or reproduce. If we consider
these facts we realize that the mere laws of chance are adequate to
account for the fact of the apparently purposeful adaptations; as they
are adequate to account for the Mendelian numbers.
CHAPTER XIII
EVOLUTION
Darwin’s work has been compared to that of Copernicus and Galileo
inasmuch as all these men freed the mind from the incubus of
Aristotelian philosophy which, with the efficient co-operation of the
church and the predatory system of economics, caused the stagnation,
squalor, immorality, and misery of the Middle Ages. Copernicus and
Galileo were the first to deliver the intellect from the idea of
a universe created for the purpose of man; and Darwin rendered a
similar service by his insistence that accidental and not purposeful
variations gave rise to the variety of organisms. In this struggle for
intellectual freedom the names of Huxley and Haeckel must be gratefully
remembered, since without them Darwin’s idea would not have conquered
humanity.
Darwin assumed that the small fluctuating variations could accumulate
to larger variations and thus cause new forms to originate.
It was the merit of de Vries[292] to have pointed out that fluctuating
variations are not hereditary and hence could not have played the rôle
assigned to them by Darwin, while discontinuous variations as they
appear in the so-called “sports” or mutations are inherited. This was
an important step in the history of the theory of evolution. It did not
touch the foundation of Darwin’s work, namely the substitution of the
idea of an accidental evolution for that of a purposeful creation; it
only modified the conception of the possible mechanism of evolution.
According to de Vries, there are special species or groups of species
which are in a state of mutation. He considers the evening primrose
on which he made his observations as one of these forms. Morgan and
his pupils have observed over 130 mutations in a fly _Drosophila_.
From our present limited knowledge we must admit the possibility that
the tendency toward the production of mutants is not equally strong
in different forms. Whether this part of de Vries’s idea is or is not
correct there can be no doubt that variations occur which consist
in the loss and apparently, though in rarer cases, in the gain or a
modification of a Mendelian factor. If we wish to visualize the basis
of such a change we may do so by imagining well-defined chemical
constituents in one or more of the chromomeres undergoing a chemical
change.
[292] de Vries, H., _The Mutation Theory_, translated by Farmer, J. B.,
and Darbishire, A. D., Chicago, 1909. _Species and Varieties_. Chicago,
1906. _Gruppenweise Artbildung_. Berlin, 1913.
This way of looking at the origin of variation has had the effect of
putting an end to the vague speculations concerning the evolution of
one form from another. We demand today the experimental test when such
a statement is made and as a consequence the amount of mere speculation
in this field has diminished considerably.
It is possible that any further progress concerning evolution must come
by experimental attempts to bring about at will definite mutations.
Such attempts have been reported but they are not all beyond the
possibility of error.[293] The most remarkable among them are those by
Tower who by a very complicated combination of effects of temperature
and moisture claims to have produced definite mutations in the potato
beetle. The conditions for these experiments are so expensive and
complicated that a repetition by other investigators has not yet been
possible.
[293] For a critical discussion of the details, see Bateson, W.,
_Problems of Genetics_, New Haven, 1913, Chapter X.
It is, however, still uncertain whether the mere addition or loss of
Mendelian characters can lead to the origin of new species. Species
specificity is determined by specific proteins (Chapter III.), while
some Mendelian characters at least seem to be determined by hormones or
substances which need neither be proteins nor specific for the species.
CHAPTER XIV
DEATH AND DISSOLUTION OF THE ORGANISM
1. It is an old saying that we cannot understand life unless we
understand death. The dead body, if its temperature is not too low
and if it contains enough water, undergoes rapid disintegration. It
was natural to argue that life is that which resists this tendency to
disintegration. The older observers thought that the forces of nature
determined the decay, while the vital force resisted it. This idea
found its tersest expression in the definition of Bichat, that “life
is the sum total of the forces which resist death.” Science is not the
field of definitions, but of prediction and control. The problem is:
first, how does it happen that as soon as respiration has ceased only
for a few minutes the human body is dead, that is to say, will commence
to undergo disintegration, and second, what protects the body against
this decay while the respiration goes on, although temperature and
moisture are such as to favour decay?
The earlier biologists had already raised the question why it was that
the stomach and intestine did not digest themselves. The hydrochloric
acid and the pepsin in the stomach and the trypsin in the intestine
digest proteins taken in in the form of food; why do they not digest
the proteins of the cells of the stomach and the intestine? They will
promptly digest the stomach as soon as the individual is dead, but
not during life. A self-digestion may also be caused if the arteries
of the stomach are ligatured. Claude Bernard and others suggested
that the layer of mucus protected the cells of the stomach and of the
intestine from the digestive enzymes; or that the epithelial layer
had a protective effect. Pavy suggested that the alkali of the blood
had a protective action. All these theories became untenable when
Fermi showed that all kinds of living organisms, protozoans, worms,
arthropods, are not digested in solutions of trypsin as long as they
are alive, while they are promptly digested in the same solution
when dead.[294] This is in harmony with the fact that many parasites
live in the intestine without being digested as long as they are
alive. Fermi concluded that the living cell cannot be attacked by the
digestive ferments, while with death a change occurs by which they
can be attacked. But what is this change? Fermi seems to be inclined
to think that the “living molecule” of protein is not hydrolysable
(perhaps because the enzyme cannot attach itself to it?), while a
change in the constitution or configuration of the proteins takes place
after respiration has ceased. The fact that the living cell resists
the digestive action of trypsin and pepsin has found two other modes
of explanation, first, that the cells are surrounded by a membrane or
envelope through which the enzyme cannot diffuse, and second, that the
living cells possess antiferments. But the so-called antiferments are
also said to exist after the death of the cell, whereas after death
the cell is promptly digested. Frédéricq, as well as Klug, has shown
that worms which are not attacked by trypsin are digested by this
enzyme when they are cut into small pieces; although the pieces of
course contain the antienzyme. The other suggestion that a membrane
impermeable for trypsin protects the cells would explain why living
protozoa are not digested by trypsin, but it leaves another fact
unexplained, namely, the autodigestion of all the cells after death by
enzymes contained in the cells themselves.
[294] Fermi, C., _Centralbl. f. Bacteriologie_, Abt. 1, 1910, lvi., 55.
2. The disintegration of the body after death is not caused exclusively
or even chiefly by the digestive enzymes of the intestinal tract or
the micro-organisms entering the dead body from the outside, but by
the enzymes contained in the cells themselves. This phenomenon of
autolysis[295] was first characterized by Hoppe-Seyler.[296]
All organs suffering death within the organism, in the absence of
oxygen, undergo softening and dissolution in a manner resembling
that of putrefaction. In the course of that process, albuminous
matter gives rise to leucin and tyrosin, fat to free acids and
soaps. This maceration, identical with the pathological conception
of softening, is accomplished without giving rise to ill odour and
is a process similar to the one resulting from the action of water,
acids, and digestive enzymes.
[295] Levene, P. A., _Autolysis_. The Harvey Lectures, 1905-1906, p.
73, gives a full account of the work on this subject up to 1905.
[296] Hoppe-Seyler, F., _Tübinger med.-chem. Untersuchungen_, 1871, P.
499.
In work of this kind, rigid asepsis is required to exclude the
possibility of bacterial infection and this was first done by
Salkowski, who showed that in aseptically kept tissues like liver and
muscle the amount of substances that can be extracted with hot water
increases considerably. By the work of others, especially Martin Jacoby
and Levene, it was established that the power of self-digestion is
shared by all organs. Analysis of the products of the autodigestion
of tissues shows that, _e. g._, the amino acids, which constitute the
proteins, are produced. Dakin, Jones, and Levene demonstrated the
hydrolytic products of the nucleins, in the case of the self-digestion
of tissues.[297]
[297] Levene, P. A., _Am. Jour. Physiol._, 1904, xii., 276.
Again the question arises: Why do the tissues not undergo autolysis
during lifetime and what protects them, and the answer is that
self-digestion is a consequence of the lack of oxidations. The presence
of antiferments must continue after death and cannot be the cause which
prevents the self-digestion during life, since nothing indicates the
destruction of the hypothetical antidigestive enzymes through lack of
oxygen. The recent work of Bradley and Morse[298] and of Bradley[299]
has thrown some light on the problem. These authors found that proteins
of the liver which are indigestible can be made digestible by the liver
enzymes if an acid salt or a trace of acid is added to the mixture.
A m/200 HCl solution gives marked acceleration of the autodigestion
of the liver. This would explain why autodigestion takes place after
oxidations cease. In many if not all the cells, acids are constantly
formed during lifetime, _e. g._, lactic acid, which through oxidation
are turned to CO₂, and this diffuses into the blood so that the H ion
concentration in the cells does not rise materially. If, however,
the oxidations cease, as is the case after death, the formation of
lactic acid continues, but the acid is not oxidized to CO₂ and thus
removed, and as a consequence the H ion concentration increases in the
cells and the self-digestion of proteins, which the digestive enzymes
contained in the cells themselves could not attack formerly, becomes
possible. Acid increases the digestibility of a protein, probably by
salt formation. Theoretically we should not be surprised that while
in the liver an increase in the C_{H} favours autolysis in other
tissues the same result is produced by the reverse effect. We might
say that the preservation of a certain C_{H} probably at or near the
point of neutrality during life prevents self-digestion, while the
gross alteration of the C_{H} in either direction after death (or
after the cessation of oxidations in the tissues) induces autolysis.
Bradley indeed suggests that many of the phenomena of autolysis during
lifetime, such as atrophy, necrosis, involution, might be due to an
increase in the C_{H} in the tissues.
[298] Bradley, H. C., and Morse, M., _Jour. Biol. Chem._, 1915, xxi.,
209.
[299] Bradley, H. C., _ibid._, 1915, xxii., 113.
These facts agree with the suggestion of Fermi that in the living
cell the proteins cannot be attacked by the digestive enzymes but
relieves us of the necessity of making the monstrous assumption of
a “living molecule” of proteins as distinct from a “dead” molecule.
The difference between life and death is not one between living and
dead molecules, but more likely between the excess of synthetic over
hydrolytic processes.