Explain why a specimen must be viewed under a compound light microscope?

Because it is very small.

Explanation:

Specimens observed by a compound microscope needs to be very thin, so that light can pass through them. If they were not slim enough for light to cross by them, they would not be apparent under a compound microscope.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".