[258] Loeb, J., and Ewald, W. F., _Biochem. Ztschr._, 1913, lviii., 179.
---------------+-------------------------------------------- _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 CO{ 2} in seeds[259] and the assimilation of CO{ 2} 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_.