CHAPTER V 



THE INHERITANCE OF ACQUIRED CHARACTERS 



In the preceding chapters we have seen that external factors 

 may cause definite changes in the form, color, markings, etc., 

 of animals, as well as certain changes in plants. The question 

 arose whether these effects are transmitted to the offspring of 

 the next generation. An examination of the evidence seemed to 

 show that in most cases the effects are inherited only in so far 

 as the germ-cells are also affected by the external factors. 



It has been long recognized that internal factors also may 

 cause changes in animals. The use of an organ may increase 

 its size, and also its effectiveness, even when the change in form 

 is so slight as to escape notice. Here, also, the question arises 

 whether these effect'^ are inherited. Disease or injury may 

 bring about changes in an organ, and again the question has 

 been raised as to whether the effects are transmitted to the 

 offspring. 



The phrase ''the inheritance of acquired characters" is used 

 to include supposed cases of inheritance of these different kinds 

 of effects; and it is customary to use this term to include also 

 cases like those described in the last chapter, in which the ex- 

 ternal agents act directly on the germ-cells, as well as on the 

 body-cells. In this chapter I shall use the term in a restricted 

 sense, and include under it only those cases in which the body- 

 cells are first affected and are then supposed to transmit their 

 influence to the germ-cells. 



It is not necessary to consider at length the historical origin 

 of the idea that acquired characters are inherited. It is well 

 known that Lamarck based his theory of evolution on this 



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