Effect of temperature on development 259 



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 pupae 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 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. hydro- 

 pses) 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°. 

 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 (6), it is not to be expected that this same perfect balance 

 will be maintained for extremely high or extremely low tempera- 

 tures ; 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 



17—2 



