 Quine opens his discussion of natural kinds by asking “what tends to confirm an induction?” (see Criterion 2 above) (1969, 114). With Goodman’s new riddle of induction and Hempel’s paradox of confirmation in mind, Quine answers that it is the similarity or sameness of kinds between instances that permits an induction: two green emeralds are more similar than two grue emeralds when one of them is green and the other blue. It is the “dubious scientific standing of a general notion of similarity, or of kind” (1969, 116) that prompts the discussion alluded to above. An intuitive notion of similarity is used in our ordinary inductions. And the force of Darwinian processes gives us some reason to think that we have evolved an innate similarity space that corresponds to some natural similarities. At the same time, natural science, which developed from these primitive inductions, supported by similarities, is able to reveal deeper similarities that may contradict superficial similarities. For example, we have strong sense that things of the same colour are objectively alike, but science tells us that there is no deep similarity among like-coloured things. Quine thinks that in due course science will obviate the need for a general notion of similarity or kind: in each area of science more specific notions will take the place of the generic notion; this is a sign of the maturity of a branch of science. For example, in zoology we may replace talk of the similarity between two animals by discussion of the historical proximity of their closest common ancestor.