CHAPTER XII 



HEREDITY AND DEVELOPMENT 



" To think that heredity will build up organic beings without mechanical 

 means is a piece of unscientific mysticism." — Wilhelm His. But would 

 even an omniscience of mechanical means explain the facts ? 



§ I. Theories of Development 



§ 2. Weismann's Theory of the Germ-Plasm 



§ 3. Note on Rival Theories 



§ 4. Weismann's Theory of Germinal Selection 



§ I. Theories of Development 



The Secret of Development. — In his forty-ninth exercitation 

 on the " efficient cause of the chicken," Harvey (1578-1657), 

 quaintly expressed his bewilderment before the baffling problem 

 of development. " Although it be a known thing subscribed 

 by all, that the foetus assumes its original and birth from the 

 male and female, and consequently that the egge is produced 

 by the cock and henne, and the chicken out of the egge, yet 

 neither the schools of physicians nor Aristotle's discerning brain 

 have disclosed the manner how the cock and its seed doth 

 mint and coine the chicken out of the egge." How much nearer 

 a disclosure are we to-day ? The visible sequences in the 

 process of development are in many cases familiar, the external 

 conditions of development are in many cases well known, and 

 we have a little insight in regard to what is called the mechanics 

 of development ; but, on the whole, we have to confess that we 



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