(a) THE INFLUENCE OF TEMPERATURE UPON THE DENSITY OF PELAGIC ORGANISMS
AND THE DURATION OF LIFE.
It has often been noticed by explorers who have had a chance to compare
the faunas in different climates that in polar seas such species as
thrive at all in those regions occur, as a rule, in much greater density
than they do in the moderate or warmer regions of the ocean. This refers
to those members of the fauna which live at or near the surface, since
they alone lend themselves to a statistical comparison. In his account
of the Valdivia expedition, Chun (Chun, "Aus den Tiefen des Weltmeeres",
page 225, Jena, 1903.) calls especial attention to this quantitative
difference in the surface fauna and flora of different regions. "In the
icy water of the Antarctic, the temperature of which is below 0 deg C.,
we find an astonishingly rich animal and plant life. The same condition
with which we are familiar in the Arctic seas is repeated here, namely,
that the quantity of plankton material exceeds that of the temperate and
warm seas." And again, in regard to the pelagic fauna in the region of
the Kerguelen Islands, he states: "The ocean is alive with transparent
jelly fish, Ctenophores (Bolina and Callianira) and of Siphonophore
colonies of the genus Agalma."
The paradoxical character of this general observation lies in the fact
that a low temperature retards development, and hence should be
expected to have the opposite effect from that mentioned by Chun. Recent
investigations have led to the result that life-phenomena are affected
by temperature in the same sense as the velocity of chemical reactions.
In the case of the latter van't Hoff had shown that a decrease in
temperature by 10 degrees reduces their velocity to one half or less,
and the same has been found for the influence of temperature on the
velocity of physiological processes. Thus Snyder and T.B. Robertson
found that the rate of heartbeat in the tortoise and in Daphnia is
reduced to about one-half if the temperature is lowered 10 deg C., and
Maxwell, Keith Lucas, and Snyder found the same influence of temperature
for the rate with which an impulse travels in the nerve. Peter observed
that the rate of development in a sea-urchin's egg is reduced to less
than one-half if the temperature (within certain limits) is reduced by
10 degrees. The same effect of temperature upon the rate of development
holds for the egg of the frog, as Cohen and Peter calculated from
the experiments of O. Hertwig. The writer found the same
temperature-coefficient for the rate of maturation of the egg of a
mollusc (Lottia).
All these facts prove that the velocity of development of animal life
in Arctic regions, where the temperature is near the freezing point of
water, must be from two to three times smaller than in regions where the
temperature of the ocean is about 10 deg C. and from four to nine times
smaller than in seas the temperature of which is about 20 deg C. It is,
therefore, exactly the reverse of what we should expect when authors
state that the density of organisms at or near the surface of the ocean
in polar regions is greater than in more temperate regions.
The writer believes that this paradox finds its explanation in
experiments which he has recently made on the influence of temperature
on the duration of life of cold-blooded marine animals. The experiments
were made on the fertilised and unfertilised eggs of the sea-urchin, and
yielded the result that for the lowering of temperature by 1 deg C.
the duration of life was about doubled. Lowering the temperature by 10
degrees therefore prolongs the life of the organism 2 to the power 10,
i.e. over a thousand times, and a lowering by 20 degrees prolongs it
about one million times. Since this prolongation of life is far
in excess of the retardation of development through a lowering of
temperature, it is obvious that, in spite of the retardation of
development in Arctic seas, animal life must be denser there than in
temperate or tropical seas. The excessive increase of the duration of
life at the poles will necessitate the simultaneous existence of more
successive generations of the same species in these regions than in the
temperate or tropical regions.
The writer is inclined to believe that these results have some bearing
upon a problem which plays an important role in theories of evolution,
namely, the cause of natural death. It has been stated that the
processes of differentiation and development lead also to the natural
death of the individual. If we express this in chemical terms it means
that the chemical processes which underlie development also determine
natural death. Physical chemistry has taught us to identify two chemical
processes even if only certain of their features are known. One of
these means of identification is the temperature coefficient. When two
chemical processes are identical, their velocity must be reduced by
the same amount if the temperature is lowered to the same extent.
The temperature coefficient for the duration of life of cold-blooded
organisms seems, however, to differ enormously from the temperature
coefficient for their rate of development. For a difference in
temperature of 10 deg C. the duration of life is altered five hundred
times as much as the rate of development; and, for a change of 20 deg
C., it is altered more than a hundred thousand times as much. From this
we may conclude that, at least for the sea-urchin eggs and embryo,
the chemical processes which determine natural death are certainly not
identical with the processes which underlie their development. T.B.
Robertson has also arrived at the conclusion, for quite different
reasons, that the process of senile decay is essentially different from
that of growth and development.
(b) CHANGES IN THE COLOUR OF BUTTERFLIES PRODUCED THROUGH THE INFLUENCE
OF TEMPERATURE.
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 pupae
of Vanessa prorsa several weeks at a temperature of from 0 deg to 1 deg
Weismann succeeded in obtaining from the summer chrysalids specimens
which resembled the winter variety, Vanessa levana.
If we wish to get a clear understanding of the causes of variation in
the colour and pattern of butterflies, we must direct our attention to
the experiments of Fischer, who worked with more extreme temperatures
than his predecessors, and found that almost identical aberrations
of colour could be produced by both extremely high and extremely low
temperatures. This can be clearly seen from the following tabulated
results of his observations. At the head of each column the reader
finds the temperature to which Fischer submitted the pupae, and in the
vertical column below are found the varieties that were produced. In the
vertical column A are given the normal forms:
(Temperatures in deg C.)
0 to -20 0 to +10 A. +35 to +37 +36 to +41 +42 to +46
(Normal forms)
ichnusoides polaris urticae ichnusa polaris ichnusoides
(nigrita) (nigrita)
antigone fischeri io - fischeri antigone
(iokaste) (iokaste)
testudo dixeyi polychloros erythromelas dixeyi testudo
hygiaea artemis antiopa epione artemis hygiaea
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 deg C.) are absolutely identical with
the aberrations produced by exposing the pupae to extremely high
temperatures (42 to 46 deg C.). Moreover the aberrations produced by a
moderately low temperature (from 0 to 10 deg C.) are identical with the
aberrations produced by a moderately high temperature (36 to 41 deg 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 physico-chemical
point of view, we must realise 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. oxydations). 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 deg. But
we know also that the temperature coefficients do not vary equally from
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. Lack of space precludes an account of these important
experiments, as of so many others.