226 Heredity and Environment 



tiation has already begun before the relation of a part to surround- 

 ing parts has been changed, it may continue to differentiate as if 

 no change of position or relation had taken place. Thus if a right 

 limb is transplanted to the left side of the body after it has begun 

 to differentiate it remains a right limb and is not modified by its 

 new relations (Harrison) ; if the cleavage cells are already dif- 

 ferentiated in the four-celled stage, each cell when separated from 

 the others will give rise to only one-quarter of an animal. In 

 short the organ or cell is already set, or fixed, or differentiated 

 to such an extent that it can not change its fate even though 

 its environment should change. Such cases are known as "self- 

 differentiation." 



Many students of the physiology of development have been 

 led to the view that the fundamental causes of development are to 

 be found not in the egg cell itself but in environmental stimuli 

 and in the interaction of the various parts. Driesch in particular 

 regards the egg, or any cleavage cell, as an "harmonic equipoten- 

 tial system," that is, any part is capable of any fate and its actual 

 fate is determined by its relation to other parts; in the striking 

 phrase of Driesch, "The fate of a part is a function of its posi- 

 tion." We now know that this expresses only a fraction of the 

 truth. The fate of a part is primarily a function of its organiza- 

 tion and only secondarily a function of its position. 



These are only a few illustrations of the many kinds of abnor- 

 mal development which may be caused by changed environment or 

 by unusual functional activities. At all stages of ontogeny the 

 course of development may be altered by extrinsic stimuli but 

 earlier stages may be more profoundly influenced than later ones. 



D. INHERITANCE OR NON-INHERITANCE OF 



ACQUIRED CHARACTERS 



Few questions in biology have been discussed so fully and so 

 fruitlessly as this. It is a problem of the greatest interest not 

 only to students of biology but also to sociologists, educators and 

 philanthropists and yet it is still to a certain extent an unsolved 

 problem. 



