BIOLOGY IN HUMAN AFFAIRS 



more likely to stop altogether than to continue in an 

 abnormal manner. 



These general considerations give us a clearer idea of 

 the processes collectively known as growth; but, after all, 

 they do not answer the question of greatest interest. We 

 want to know precisely what variations can be expected — 

 due to the deviations in environmental factors that are 

 ordinarily confronted during development — in a series of 

 individuals having identical gene patterns. We may admit 

 that the gene equipment makes it possible for an egg to 

 develop into a potato, or a fish, or a man, as the case may 

 be, with the appropriate characteristics which make these 

 organisms distinguishable from one another. We may admit 

 that the genetic constitution even marks out rather defi- 

 nitely what kind of potato or fish or man shall develop. 

 But what are the extreme developmental variations attrib- 

 utable to the environment as a whole? 



It is difficult to give an answer that will cover all cases, 

 for various classes of organisms have different powers of 

 adjustment and different factors of sensitivity. The type of 

 plant or animal concerned must therefore be taken into 

 consideration. In general, capacity for adjustment dimin- 

 ishes and sensitivity increases as we go up in the evo- 

 lutionary scale. Thus the lower orders of plants are less 

 sensitive than the higher orders; aquatic animals are less 

 impressionable than land animals; cold-blooded animals 

 are less responsive than warm-blooded animals. The mam- 

 mals, and in particular man, are extremely sensitive. These 

 facts give rise to an interesting situation. Experimental 

 alteration of the environment can cause greater effects on 

 the lower organisms than on the higher organisms simply 

 because the latter have developed natural devices which 

 preclude any effective modification of conditions. It is not 

 to be assumed, however, that even the lower organisms 

 have unlimited powers of response. They are able to 



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