We are not told in so many words, but have to pick up the implied answer by osmosis. Jacobs is enthralled and persuaded by the way in which evolution creates new species, and alters organic form within species. Generally, she says, a specialized form emerges from a more general form because it provides a better adaptation to the local and current environment. This progressive differentiation is an ongoing process, not a one-time event. Whole collections of species co-evolve, changes in some providing the opportunity for, and the payoff to, adaptive changes in others.
Species form a developing web.
One can see why Jacobs is so delighted by this way of describing evolutionary development. This is certainly one important, interesting, and satisfying route to economic development. If this is what economies do, then presumably economists ought to study this process. But here a problem arises that does not seem to occur to Jacobs and her characters. This sort of naturalist thinking by analogy has built-in limitations.