Why is darwin’s theory of evolution by natural selection not ‘just a theory? ’

If a set of instructions that determines all of the charactersitics of an organism is comparedto a book,and a chromosme is compared to chaper in the book,then what might be compared to a paragraph in the book?

Indica el nombre y la fucion de los vasos sanguineos que entran y salen de los riñones

Which would be the most useful source of evidence to support mcneill's contention?