Questions 1-8 refer to the following passage from a biology textbook.

Mammals, like all other creatures, (1).... to their environment or perish. Adaptation (2).... when an animal with traits suited to survival produces offspring. Conversely, an organism with disadvantageous or harmful traits (3).... fewer or no offspring. This is the basis of Darwin's theory of evolution. In his book On the Origin of Species, which (4).... this theory, Darwin wrote: "The preservation of favorable individual differences and variations and the destruction of those which are injurious, I (5).... Natural Selection or the Survival of the Fittest." By "fittest," Darwin (6).... the largest, smartest, or strongest members of a group. Instead, he (7)... the term to refer to those members of a species that produce the largest number of surviving offspring. If a species cannot leave offspring that can adapt to an environment, that species (8)....