In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives.

_______________ the course of the development of different life-forms heredity—which, in plain English, is unconscious memory generated in the first life-form and ______________ through all the different species—is the sole factor in the ____________ of the parent properties; while adaptation to surrounding conditions and circumstances, natural selection in the struggle _____________ existence, and partner selection in the struggle of the males for ____________ are the principal factors in the differentiation of species.