Attacks the doctrine of "heterogeny" (spontaneous generation during each geological period) as completely lacking in evidence.

Defends natural selection as connecting large classes of facts in natural history. That certain forms have not changed since remote epochs is not an objection of any force.

Replies to a reviewer’s statement, that any theory of descent will connect large classes of facts, by pointing out that no other explanation has been as satisfactory as natural selection. But whatever view is adopted "signifies extremely little in comparison with the admission that species have descended from other species and have not been created immutable".