 Ariew (2007) has proposed a second, very different way in which canalization might offer an advantage over the invariance account. Ariew suggests that the concept of canalization provides a framework for comparing species in their sensitivity to environmental factors. For example, Sober 1998 introduces three different types of learning exhibited by different species of songbird. Type 1 songbirds produce species-typical song even if the bird is reared in silence. Type 2 birds produce species-typical song only after sessions of call and response with a “tutor” from their own species. Type 3 songbirds produce their species-typical song after encountering any song at all. These cases are presented by Ariew (2007) as if they vary along a continuum of sensitivity to environmental cues. Type 1 learning seems the least sensitive. Type 3 learning is only slightly more sensitive that Type 1, since the trait is “triggered” by exposure to a wide range of songs. Type 2 learning is the most sensitive to the specific songs that a bird encounters. Hence, Ariew’s suggestion is that Type 1 is “more innate” than Type 3, which is in turn “more innate” than Type 2. By suggesting that birdsong can be organized along a single axis of innate/triggered/acquired, Ariew seems to be proposing a form of explanatory unification in the sense developed by Kitcher (1989) (see the entry on  Scientific Explanation).  An important feature of explanatory unification is the capacity to show how disparate phenomena, previously thought to be unrelated, can be derived using a single, limited vocabulary for describing them. We can apply this to Ariew’s birdsong example. Upon first encountering the three types of learning (Types 1, 2, and 3), one might assume that they are entirely distinct phenomena. However, to further discover that they vary along a single parameter – i.e., the “innateness/triggering/acquired” dimension – would seem to unify them, in Kitcher’s sense.