392 THE WONDER OF LIFE 



substance ' or plastosome, a seed to which the surrounding 

 cells supply the appropriate environment and the necessary 

 liberating stimuli. Above all, we must not think of the 

 matter too simply, too mechanically. That mechanical 

 factors operate directly on the developing embryo will 

 be admitted by all. There are bound to be pressures and 

 tensions and the like, which make themselves felt. But it 

 has to be borne in mind that the essential process is the 

 active expression of an inconceivably intricate organiza- 

 tion, which has been gradually wrought out through tens 

 of thousands of years. When Professor His maintained that 

 the large eyes of the young chick are the direct cause, by 

 compression, of the sharp beak of the bird, he was taking 

 too simple a view of the problem, and mistaking the cart 

 for the horse. 



To take a concrete illustration of the absolutely essential 

 influence of the environment. It is well known that the 

 absence of the appropriate temperature at a critical period 

 may have a profound effect. It may arrest cell-divisions in 

 one part of the embryo more than in another, and strange 

 aberrations may result. Or it may operate by hindering 

 the operation of certain ferments. Thus Dr. J. Dewitz 

 placed the nests of a wasp (Polistes) in a refrigerator for 

 forty-eight hours, and found that this had the effect of 

 hindering the development of the wings in the pupae. 

 Similar experiments with the pupae of the blow-fly (Calli- 

 phdra) also resulted in defective wings. Again we are made 

 to feel that each stage in development has its appropriate 

 external nurture. 



Environment affords or denies stimulus, and according 

 to the liberality or parsimony will be, in many cases, the 

 degree of development attained by the animal. A diagram- 



