THE INFLUENCES OF NURTURE 91 



induced there were not only decreases and increases in 

 what was already present ; there were some distinct 

 novelties which maintained their distinctiveness when 

 crossed with the parental strains. It is probable that 

 the injections into the ovaries acted as variational stimuli 

 on the germ-cells (see lecture on *' The Direct Influence 

 of Environment," in Fifty Years of Darwinism, 1919). 



Facts like those cited should be borne in mind in 

 connection with Man and ordinary placental mammals 

 where the unborn offspring lives in intimate partner- 

 ship (or symbiosis) with its mother. Slight changes 

 produced in the blood of the mother by peculiarities 

 of nurture may affect the development of the offspring. 

 It is very important to realise the difficulty of distin- 

 guishing between what is due to inherited nature (and 

 average normal nurture) and what is due to sorYie pecu- 

 liarity in ante-natal nurture. 



Here we may consider the parable of the Planarian 

 worms (Planaria), very simple ' living films/ often 

 seen gliding about by means of microscopic lashes (cilia) 

 on the pond-weeds. When they are allowed no choice 

 of food, but restricted to the uncongenial flesh of fresh- 

 water mussels, they cease to thrive. Their vital pro- 

 cesses, Professor Child tells us, are slowed down, and 

 their resistance-powers to deleterious influences are 

 lessened. They become old, and, what is more remark- 

 able, if the diet be continued for several generations, 

 they begin to be born old. Now one does not dream of 

 arguing from worm to man, but there is a suggestion here 

 of the danger of too much porridge at the one extreme 



