I begin from _Enumerative Suggestion_ drawn from observation of Butterflies.
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.
2. The non-resembling species in the genus _x_ show no initial tendency towards _y_.
∴ The non-resemblances observed are such as could not produce protective resemblances. This is a formally bad argument from two negative premisses justified by its positive meaning, which implies that _just where_ the alleged effect ceases, the alleged cause ceases too.
If you look at the case in the Natural History Museum [1] you see the normal Pierinae down one side, not approaching Euploinae. They are the positive examples, negatively confirming the explanation of those which do approach Euploinae. These latter all start from some form which varied slightly, by accident we presume, towards Euploinae, and then this partially resembling series splits into three sets, each leading up to a different and complete protective resemblance.