Question

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What was the preformation theory for inheritance?

What could the preformation theory not explain?

Answer 1

What was the preformation theory for inheritance?

Theory of preformation: a theory that an individual develops by simple enlargement of a tiny fully formed organism (a homunculus) that exists in the germ cell.
The theory states that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, in real terms, prior to their development. It suggests that all organisms were created at the same time, and that succeeding generations grow from homunculi, or animalcules, that have existed since the beginning of creation.

What could the preformation theory not explain?

Despite careful observation of developing embryos, epigenesis suffered from a lack of a theoretical mechanism of generation. Wolff proposed an "essential force" as the agent of change, and Immanuel Kant with Johann Friedrich Blumenbach proposed a "developing drive" or Bildungstrieb, a concept related to self-organization.

Naturalists of the late 18th century and the 19th century embraced Wolff's philosophy, but primarily because they rejected the application of mechanistic development, as seen in the expansion of miniature organisms. It was not until the late 19th century that preformationism was discarded in the face of cell theory. Now, scientists "realized that they need not treat living organisms as machines, nor give up all hope of ever explaining the mechanisms that govern living beings."

When John Dalton's atomic theory of matter superseded Descartes' philosophy of infinite divisibility at the beginning of the 19th century, preformationism was struck a further blow. There was not enough space at the bottom of the spectrum to accommodate infinitely stacked animalcules, without bumping into the constituent parts of matter.

Answer 2

Used sources:
https://en.wikipedia.org/wiki/Preformationism#Criticisms_and_cell_theory

http://www.thefreedictionary.com/theory+of+preformation

Other potentially helpful resources:
http://www.britannica.com/science/preformation-theory

http://plato.stanford.edu/entries/epigenesis/