What is the mechanism by which bulk flow occurs at the capillaries?

Relatively higher hydrostatic pressure on the arterial end of the capillary and relatively higher colloid oncotic pressure on the venous end results in net flow out of the capillary.

Explanation:

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".