Doctrine of signatures: (above) a plant (fennel) with seed-pods resembling the crest of a bird, and (below) a bird's head. Coloured ink drawing by C. Etheridge, 1906, after G.B. Della Porta.

In his Phytognomica, Della Porta discerns the hidden qualities of plants from the analogy of their forms with comparable features of animals. Those features of animals are "signs" which signify the same effects in plants which they have in animals: "ut cui signum competat, eidem et effectus: et cui effectio, pariter & signum: & quae signis vacent, & effectibus vacent" (Phytognomica, lib I, cap. xiii, Rouen 1650, p. 26); in English "That to which a sign belongs, to it also the effect belongs; and that to which the effect belongs, equally to it the sign belongs; and those which lack the signs also lack the effects"