按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
n 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。
IV。 THE EFFECTS OF LIGHT。
At the present day nobody seriously questions the statement that the action of light upon organisms is primarily one of a chemical character。 While this chemical action is of the utmost importance for organisms； the nutrition of which depends upon the action of chlorophyll； it becomes of less importance for organisms devoid of chlorophyll。 Nevertheless； we find animals in which the formation of organs by regeneration is not possible unless they are exposed to light。 An observation made by the writer on the regeneration of polyps in a hydroid； Eudendrium racemosum； at Woods Hole； may be mentioned as an instance of this。 If the stem of this hydroid； which is usually covered with polyps； is put into an aquarium the polyps soon fall off。 If the stems are kept in an aquarium where light strikes them during the day； a regeneration of numerous polyps takes place in a few days。 If； however； the stems of Eudendrium are kept permanently in the dark； no polyps are formed even after an interval of some weeks； but they are formed in a few days after the same stems have been transferred from the dark to the light。 Diffused daylight suffices for this effect。 Goldfarb； who repeated these experiments； states that an exposure of comparatively short duration is sufficient for this effect； it is possible that the light favours the formation of substances which are a prerequisite for the origin of polyps and their growth。
Of much greater significance than this observation are the facts which show that a large number of animals assume； to some extent； the colour of the ground on which they are placed。 Pouchet found through experiments upon crustaceans and fish that this influence of the ground on the colour of animals is produced through the medium of the eyes。 If the eyes are removed or the animals made blind in another way these phenomena cease。 The second general fact found by Pouchet was that the variation in the colour of the animal is brought about through an action of the nerves on the pigment…cells of the skin； the nerve…action being induced through the agency of the eye。
The mechanism and the conditions for the change in colouration were made clear through the beautiful investigations of Keeble and Gamble； on the colour…change in crustaceans。 According to these authors the pigment…cells can； as a rule； be considered as consisting of a central body from which a system of more or less complicated ramifications or processes spreads out in all directions。 As a rule； the centre of the cell contains one or more different pigments whic