The main gap in Darwin's original theory was that he could not explain

  • A. why more offspring are produced than can survive
  • B. how variations arise and how they are passed on
  • C. the struggle for existence between individuals
  • D. the geographical distribution of related species

Explanation

Darwin had no knowledge of genes, so although he built his whole argument on heritable variation he could not say where variation came from or why it was not blended away in each generation. Mendel's work, rediscovered in 1900, supplied particulate inheritance and mutation supplied the source, and the combination became neo Darwinism. Overproduction, struggle and distribution were all matters Darwin documented well.

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