Fill in the blanks with suitable words :

Darwin explained the survival of the fittest, but not their__________.

Darwin explained the survival of the fittest, but not their arrival.

Explanation: Darwin explained that variations (changes in the genetic makeup of an organism with time) are responsible for natural selection. Nature selects the fittest on the basis of variations which help the organism to survive and get adapt to the environment. But Darwin was unable to explain the source and how these variations had arisen. Therefore, it is said that Darwin explained the survival of the fittest, but not their arrival.