996 LIFE : OUTLINES OF GENERAL BIOLOGY 



circling on for hours, obeying their instinct, unable to break the 

 spell. But everyone knows that intelligent mammals can be trained 

 to do all sorts of unusual things. 



New Modifications. — This word "modification" is applied by 

 naturalists to individual changes of structure, or of function, that 

 are directly due to some change or peculiarity in surroundings, food, 

 or habits, and that last after the inducing cause has ceased to operate. 

 If a man goes to the Tropics for some months he may become very 

 brown or freckled. This means that an unusual amount of the dark 

 pigment called melanin, familiar in negroes, is deposited in the skin. 

 There is an advantage in this, for it may save the skin from being 

 burnt in the glare. This kind of change is very common, and it is 

 called an adjustment, the point being that it does not last. After a 

 few months at home the extra pigment has been absorbed, and the 

 man is as he was. But if he has stayed thirty years in the Tropics he 

 may become so thoroughly tanned that he will remain brown after 

 he returns to England for the rest of his life. This would be called 

 a modification, or individually acquired character. It lasts after the 

 inducing cause has ceased to operate, but there is very little evidence 

 that it can be entailed on the offspring in any representative degree. 

 But this is a separate problem which we have separately discussed; 

 let us rather give some examples of modifications. 



If sheep are taken to a much colder climate they sometimes grow 

 a thicker and longer fleece — a useful protective modification. In the 

 next generation the wool is still longer and thicker, and this is 

 natural enough, since the offspring are under the influence of the 

 cold from birth onwards, while the parents experienced it only after 

 their arrival. It has not been shown that there is any further increase 

 after the first generation bom in the new country. 



Similarly, if plants are transported from a low-lying region to the 

 high Alps they may develop a thicker skin or more hair on their 

 leaves — a protective modification which their offspring may show 

 even more markedly. If the plants are taken back again to the 

 low ground the individual leaves remain as they became on the 

 mountains, but the new leaves and the leaves of the offspring may 

 be entirely of the low-ground type. The Alpine modification does not 

 last; and the same is true of the changes that follow exposure to 

 desert conditions, or the like. Some cases are between "adjustments" 

 and "modifications". 



The experimental evidence of plasticity or modifiability is often 

 very striking. As we have already mentioned, there lives in the 

 caves of Carinthia and Dalmatia a wan, blind newt, called Proteus. 

 It is about six inches long, and shows no colour except where the red 

 blood shines through the three pairs of external gills. The eyes remain 

 arrested in development and do not reach the surface. But if Proteus 

 is brought from the cave to the new environment of a well-lighted 



