CERTAIN LAWS OF VARIATION. 



197 



tinned for the remainder of the period of growth. If 

 we neglect the transition periods in these two series, 

 we see that the effect of the abnormal temperature, be 

 it negative or positive, rapidly diminishes with progress 

 in development. In the 17th hour it is not a fifteenth 

 and in the 83rd hour not a four hundredth as great as 

 in hour 2^. In two other series of observations a 

 similar kind of result was obtained; so taking together 

 all the series of observations made, by all the methods, 

 one is therefore justified in concluding that the effect 

 of temperature on the growth of the developing organ- 

 ism diminishes rapidly and regularly from the time of 

 impregnation onwards. 



These experiments in which the same condition of 

 environment at first produced an adverse effect, and 

 then a favourable one, illustrate a very important prin- 

 ciple; one which was recognised long ago by Darwin, 

 and also by Weismann. They show that the reaction 

 of an organism to its environment depends on the 

 nature of that organism. Thus with respect to the di- 

 rect action of conditions of life, " We must bear in 

 mind," says Darwin, " that there are two factors ; 

 namely, the nature of the organism and the nature of 

 the conditions. The former seems to be much the 



