Skeletons of the Mind
The skeletons of all mammals are said by biologists to be “homologous”. If you look at the skeleton of a bat, say, and compare it with the skeleton of a human or a dog, you find the same number and arrangement of bones (give or take a couple).  This is because all mammals are descended from a common ancestor, with a particular type of skeleton. What differ are the relative lengths and sizes of the bones, which are the result of evolutionary pressures operating on the descendants of the common ancestor. In fact, the skeletons of all mammals are homeomorphisms of each other—variations on the same basic theme. They are all modifications of a single skeleton type. They thus differ from other skeleton types, such as those of crustaceans, which are exoskeletons and have a quite different structure. Each broad type of organism has the same skeletal plan, but with different extensions and compressions of relative bone size (and hence function). This is not obvious to casual observation and is quite surprising: it is easy to miss the underlying commonalities. For example, the bat skeleton is homologous to the human skeleton, but the finger bones are enormously extended to form the struts of the bat’s wing. The webbing between the bat’s fingers is itself an extension of the kind of limited webbing that even human hands possess. Still, the basic anatomy is identical; the differences are quantitative not structural. This is all in keeping with the Darwinian idea that skeletons are evolved forms that preserve an original form but change that form in gradual and continuous ways.
Can we apply the same basic principles to minds? The bat provides a nice test case: do any of its psychological faculties exhibit this pattern of inherited identity and quantitative extension? Consider echolocation: the bat emits high-pitched sounds that echo off objects and uses this to calculate where the object is and how it might be moving. So the faculties involved are sound production and hearing—though exquisitely fine-tuned hearing. Even humans can manage some rudimentary echolocation: you can shout into a well and get some idea of how deep it is from the echo’s timing. Bats are just very much better at this kind of thing, though they are still using their ears—they don’t have some entirely new sense organ. Their highly developed—extended—hearing is really a variation on hearing as it occurs in other mammals, including us. Just as their fingers are enormously extended, so their hearing is enormously extended (and they have very large ears too): their fingers are adapted to support wings and their hearing is adapted to track objects in the dark. We have a basic mental “bone” modified to serve a particular purpose, as a real bone is modified to serve a particular purpose. And we can understand how natural selection may have worked on bat ears and bat brains over many generations to produce gradually improved organs.
I suggest that we think of the mind as we think of the skeleton: an identical underlying structure with species variations in size, strength, etc. All mammals have a common ancestor and that ancestor had a specific psychological make-up, which subsequent generations inherited; but then there are also divergences brought about by differing evolutionary histories, resulting in different types of mind—though all sharing the same underlying form. We can’t draw a picture of this form, as we can of a skeleton, but it is not difficult to discern its outlines: the five senses, memory, cognition, emotion, and will—all connected, as bones are connected. No doubt the mammalian psychological skeleton is more specific, differing from that of the crustacean skeleton in important ways; but different species will exhibit variations within that more specific identity. Some will have sharper vision than others, or a keener sense of smell, or a better memory, or superior problem-solving skills, or different intensities of emotion, or different motor skills. But the variation is confined within an overall architecture, though it might be very marked. The species minds are homologous, despite varying quantitatively. Nothing fundamentally new is added in a particular evolutionary line, though quantitative differences can look like qualitative ones. Minds thus obey the same evolutionary laws as bodies—skeletons, in particular.
This is not difficult to accept for related non-human species–we can see how most mammals are psychological variations on the same theme—but some may feel that the model fails to apply to the human mind, because it is so discontinuous relative to other primate minds. However, this objection fails to reckon with the existence of “missing links” between existing humans and other now-extinct hominids. No doubt there was gradual evolution over the time of these many succeeding species, leading to some marked differences between humans today and our hominid ancestors, notably with regard to language. But it is probable that extinct human species, like Neanderthals, had some form of language, and that Homo sapiens descended from earlier species closer to us symbolically than contemporary monkeys and chimps. Other mammals have communication systems, more or less “primitive”, and there must be a continuous gradient linking our language to theirs—we still have the pattern of a basic skeletal structure and variations in the form of that structure. Our symbolic capacities are like the bat’s hands: grotesquely elongated forms of the same basic thing—grotesque, that is, from some species-specific point of view. That is, some sort of homology likely underlies all mammalian communication systems—though it may be very general and abstract (consider whale, dolphin, and human language). This would be analogous to the homologies that underlie mammalian eyes and ears, among many other things.
Of course, it is possible for brand new skeletal forms to evolve, given enough time and selective pressure, but within a broad family of organisms we expect to find homology at the skeletal level, as we see clearly with mammals (indeed, with all vertebrates). My point is that we should expect as much (or as little) psychological novelty as we see skeletal novelty—on general evolutionary grounds. And I think we find just that degree of psychological identity and variation: mammals share their basic mind-plans just as they share their basic skeleton-plans. We should think of the mind as structured like the skeleton—this is a truly “naturalistic” way to understand the mind.
The mammalian skeleton obviously evolved from earlier pre-mammalian skeletons, ultimately going back to fish skeletons and beyond. That is why we observe the anatomical similarities that we do between fish and their descendants. But in the same way the minds of mammals are descended from the minds of earlier species, going back to fish; and here too there are obvious homologies, particularly with regard to the senses. Skeletons span geological time, and so do minds—with the usual sort of constancy and variation. We should not exaggerate our differences from our ancestors, or indeed our contemporaries; but we should also be alert to extreme variations from a common origin—as with the bat hand and the human hand. Just as the skeletons of different species were not created independently of each other, but stem from a common source, so the minds of different species were not created afresh for each species, but stem from a common ancestral mind. Thus we have diversity within unity, with the unity often less obvious than the diversity. The bat’s skeleton is a vivid illustration of this basic biological fact, as is its mind. And just as we find it hard to imagine hearing like a bat, so we find it hard to imagine using our hands like a bat. We have some inkling because of the underlying homology, but the degree of variation, though essentially quantitative, renders full comprehension difficult, if not impossible.