""The Biological Problem of Today: Preformation or Epigenesis? The Basis of a Theory of Organic Development"" is a book written by Oscar Hertwig in 1896. The book discusses the two competing theories of development in biology: preformation and epigenesis. Preformation is the idea that organisms develop from pre-existing structures, while epigenesis is the idea that organisms develop from a process of gradual differentiation and growth. Hertwig argues...