CERTAIN LAWS OF VARIATION. 209 



animals. One or two of the other observations show a 

 somewhat similar, though not nearly so marked, 

 result. 



If our theoretical proof of the extreme sensitiveness 

 of an organism to its environment during the earliest 

 stages of development is held to be as valid for Mam- 

 mals as the experimental proof showed it to be for 

 Echinoderms, then we are met with a fact of some prac- 

 tical importance. We are afforded a scientific support 

 of the widely held belief in the special influence of the 

 condition of the mother at the time of conception upon 

 the subsequently developing offspring. Should such 

 conception occur when the blood and tissues of the 

 mother are tainted by products of metabolism, the re- 

 sult of excessive fatigue, or by alcohol, or by the 

 products of disease, there seems no reason why a con- 

 siderable and lasting adverse influence should not be 

 produced on the growth of the offspring. Still again, 

 we must conclude that the health of the mother dur- 

 ing pregnancy, and of the offspring during their early 

 years, is of much more importance in its permanent 

 effects on the physique and constitution than the 

 health in later years. 



Effect of Other Conditions on Variability. We have 

 seen that the variability of a developing organism is far 

 from remaining constant, but what is the condition of 

 affairs in an adult organism ? At the present day it has 

 generally been more or less tacitly assumed that as a 

 rule variability remains practically constant, and that 

 external conditions, though they may affect the aver- 

 age values of the characters of an organism, do not in- 

 fluence the range of variation of these characters 



