CHAPTER V 



INHERITANCE 



Phenomena which a complete theory of inheritance must explain 

 In the present state of our knowledge it is not possible to 

 formulate a complete theory of inheritance Different kinds 

 of inheritance Mendel's experiments and theory The value 

 and importance of Mendelism has been exaggerated Domi- 

 nance sometimes imperfect Behaviour of the nucleus of the 

 sexual cell Chromosomes Experiments of Delage and Loeb 

 Those of Cudnot on mice and Castle on guinea pigs Sug- 

 gested modification of the generally-accepted Mendelian 

 formulae Unit characters Biological isomerism Biologi- 

 cal molecules Interpretation of the phenomena of variation 

 and heredity on the conception of biological molecules 

 Correlation Summary of the conception of biological 

 molecules. 



WE have seen that variations may be, 

 firstly, either acquired or con- 

 genital, and, secondly, fluctuating 

 or discontinuous. We have further 

 seen that acquired variations at all events in 

 the higher animals do not appear to be in- 

 herited, and therefore have not played a very 

 important part in the evolution of the animal 

 world. Discontinuous congenital variations or 

 mutations are the usual starting points of new 

 species. It is not unlikely that fluctuating con- 

 genital variations, although they do not appear 

 to give rise directly to new species, may play a 



