i8 



It must include a theory of variation. 



It involves a conception of the essential struc- 

 ture of living matter. 



Principles invoked to account for heredity. 



The theories in detail : — 



Democritus (b. between 494 and 460 b. c). 



Seed of animals elaborated by contributions from 

 all parts of the body. 



Aristotle (384-321 r. c). The formative influence 

 of the soul. 



Van Helmont (i 577-1644). Transmission of spir- 

 itual characters. 



Blumenbach (i 752-1 840), Needham, and others. 

 Nisus foi'mativus, vital force, etc, 



Descartes (1662). 



First attempt at a mechanical explanation of de- 

 velopment. 



Bonnet (1720-1793). Preformation and " Emboite- 

 ment." 



BuFFON (1749- 1 804). 



Organic and inorganic matter essentially dif- 

 ferent. 



Organic molecules. 



Spermatozoa not concerned in reproduction. 



Conflict of maternal and paternal molecules. 



Owen (1849). 



Continuity of germ cells. (Afterwards denied.) 



