Certain Laws of Variation. 97 



ever amount of increased growth one might have been able to effect in 

 the " normal " larvae, by exposing them to the most favourable condi- 

 tions of growth possible. Now we see that in no case did the favour- 

 able environment succeed in forcing on the growth of the larvae to that 

 of the original normal larvae, so there was always plenty of growth 

 capacity at its disposal. 



In the experiment in which the ova were kept at 12 instead of 

 22'5, there is no doubt that the larvae could have been diminished at 

 least 10 to 15 per cent, more if the conditions had only been suffi- 

 ciently unfavourable and sufficiently long continued. Thus in the 

 .above-mentioned paper it is shown* that larvae kept during the whole 

 period of development at 10, instead of about 20, are diminished in 

 size by no less than 24 per cent. 



We see, therefore, that in two of the different methods adopted for 

 acting on the larvae there was always a considerable amount of 

 growth capacity still present. This may have been true also for the 

 third method, though in this case one cannot prove it. Now we have 

 seen in the above tables that the reaction in the latest periods of 

 development was not a hundredth or even a five hundredth part of 

 that in the first hour, and hence, even admitting the growth capacity 

 was diminished, there can be no doubt whatever that the sensitiveness 

 of the organism to the environment undergoes an enormous gradual 

 diminution. 



In order to determine exactly the sensitiveness of the developing 

 ova to environment during the various stages of growth, one should 

 keep various portions of them at the normal temperature for the first 

 three, six, &c., hours, and then expose them to the abnormal tempera- 

 ture for a few hours. Then they should be transferred, for the 

 remainder of their developmental period, to the normal temperature. 

 In this way there would always be the same amount of growth 

 capacity for the environment to work upon, and so the effects obtained 

 for the various periods would accurately express the true capacity for 

 reaction. 



The Effect of other Environmental Conditions. 



It is obvious that in order to demonstrate the principle under dis- 

 cussion, almost any sufficiently powerful condition of environment 

 might have been chosen. Temperature was hit upon first as being the 

 most convenient one, but further series of experiments were made with 

 another condition also, that of salinity of the water. It has been 

 shownf that growth of the larvae in water of a certain dilution may 

 increase the size by as much as 15 '6 per cent., whilst growth in pure 



* ' Phil. Trans.,' B, 1898, p. 481. 

 f ' Phil. Trans.,' B, 1895, p. 587. 



