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 Cuénot 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. 
E 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 
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