EVOLUTION loii 



in a moist environment ; but if the flies are reared in dry environment 

 — like good Americans — then they are perfectly normal. "Every 

 character", Prof. T. H. Morgan says, "is the realised result of the 

 reaction of hereditary factors with each other and with their environ- 

 ment." With this all biologists agree. It must not be supposed, 

 however, that the influences of surroundings, food and habits always 

 act in a uniform way. Some influences are conditions of normal 

 development ; others produce structural novelties (technically called 

 "modifications"); others act as subtle stimuli, tonics or disturbers. 



Most of our precise information in regard to the changes produced 

 by peculiarities of nurture relates to individual organisms. We know 

 what increase in weight will be shown by the heart of white rats 

 condemned to take an unusual amount of exercise. We know what 



Fig. 173. 



Illustrating the Change in the Comb in Poultry under waxing (1-4) and 

 waning (5) hormone-stimulation. 



dimensions will be attained by two freshwater snails in three months, 

 if one is reared in a narrow-necked vessel, and the other in a vessel 

 with a large free surface. We know what happens to the retina of 

 goldfishes that are kept for a long time in total darkness; but 

 through lack of energy we do not know whether the offspring would 

 be any the worse of their parents' acquired blindness. 



On the other hand, it is known that peculiarities in the nurture 

 of parents may affect the general vigour of the offspring, especially 

 in cases like mammals and flowering plants where the offspring 

 remain for a considerable time sharing in the life of the parent 

 organism. In a few cases, for instance, it has been shown that the 

 germ-cells may be prejudicially affected by some poison which is 

 saturating through the body of the parent. No one, not even 

 Weismann, has denied that the offspring may be influenced in a 

 general way, for better or for worse, by deep changes in the chemical 



