CHAPTER IV 



THEORIES OF HEREDITY 



WHILE the last chapter gave us a survey of the " facts of 

 heredity," showing us in brief outline the chief phenomena 

 connected with the reproductive process, we now come 

 to the theories of heredity that is, those ideal systems 

 which have been thought out in order to interpret and 

 explain these facts. Just as the atomic theory was formu- 

 lated with the object of explaining the chemical constitu- 

 tion of matter, so a theory of heredity is intended to serve 

 as a working idea of the fundamental problems of the 

 living organism. But there is this one great drawback 

 that, while the atomic theory has been universally accepted 

 as valid, we are still face to face with a multitude of con- 

 tested theories of heredity. 



We purpose here to treat only the most important of 

 these theories, and these only in so far as they variously 

 show special bearing on the subjects we have under dis- 

 cussion. Following the same order as that previously 

 adopted, we may conveniently deal with these subjects 

 under the heads of Reproduction (including Regeneration), 

 Maturation, Fertilization, and Development. 



I. THEORIES OF REPRODUCTION (including 

 REGENERA TION). 



We have seen that reproduction takes place either by 

 simple division of the whole organism into two, or by means 

 of greater or lesser parts of the individual, which, finally 

 constituting themselves as special cells, form the germ-cells. 



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