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 effects 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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