1. Three species of genus x closely resemble three species of y.
2. The species of x would be protected by resembling y (because y is distasteful to birds).
∴ The resemblance may be a “protective resemblance,” i.e. a resemblance brought about by survival of those thus protected.
On this there naturally follows Analogy.
1. Protective resemblances naturally increase through series of species from slighter to closer resemblances.
2. The resemblances in question increase in genus x through series of species from slighter to closer resemblance to y.
∴ The resemblances in question show important signs of being protective resemblances.
When we get thus far, a single syllogism will not really represent the argument. It can only analyse with convenience a single step in inference. But now we have connected the reason of the resemblances with the whole doctrine of natural selection, the gradual approximation of the species is most striking, and we could set up a corroborative analogy on the basis of every feature and detail of these resemblances, the tendency of which would be to show that no cause or combination of causes other than that suggested is likely to account for the observed resemblances.
{157} I give a confirmatory negative analogy.
1. No protective resemblance can grow up where there is no initial tendency to resemblance.