"What pattern connects the crab to the lobster and the orchid to the primrose and all the four of them to me? And me to you? And all the six of us to the ameoba in one direction and the back-ward schizophrenic in another?"
Mind & Nature: A Necessary Unity

Homology

1. Bilateral Symmetry

"So they looked at the crab. And first of all, they came up with the observation that it is symmetrical; that is, the right side resembles the left.

Then they observed that one claw was bigger than the other. So it was not symmetrical. ...

Going back to symmetry,
somebody said that "yes, one claw is bigger than the other, but both claws are made of the same parts."

Ah! What a beautiful and noble
statement that is,
how the speaker politely flung into the trash can the idea that size could be of primary or profound importance and went after the pattern which connects. He discarded an asymmetry in size in favor of a deeper symmetry in formal relations."
Mind & Nature: A Necessary Unity

2. Serial Homology

"Later, it appeared that not only are the two claws built on the same "ground plan," (i.e., upon corresponding sets of relations between corresponding parts) but that these relations between corresponding parts extend down the series of the walking legs. We could recognized in every leg pieces that corresponded to the pieces in the claw.

And in your own body, of course, the same sort of thing is true. Humerus in the upper arm corresponds to femur in the thigh, and radius-ulna corresponds to tibia-fibula; the carpals in the wrist correspond to tarsals in the foot; fingers correspond to toes.

The anatomy of the crab is repetitive and rhythmical. It is, like music, repetitive with modulation. Indeed the direction from head toward tail corresponds to a sequence in time:
In embryology,
the head is older than the tail.
A flow of information is possible from front to rear."
Mind & Nature: A Necessary Unity

3. Phylogenetic Homology

"Professional biologists talk about phylogenetic homology for that class of facts of which one example is the formal resemblance between my limb bones and those of a horse. Another example is the formal resemblances between the appendages of a crab and those of a lobster."
Mind & Nature: A Necessary Unity

Let me start again. The parts of a crab are connected by various patterns of bilateral symmetry, of serial homology, and so on. Let us call these patterns within the individual growing crab first-order connections. But now we look at crab and lobster and we again find connection by pattern. Call it second-order connection, phylogenetic homology.

Now we look at man or horse and find that, here again, we can see symmetries and serial homologies. When we look at the two together, we find the same cross-species sharing of pattern with a difference (phylogenetic homology). And, of course, we also find the same discarding of magnitudes in favor of shapes, patterns, and relations. In other words, as this distribution of formal resemblances is spelled out, it turns out that gross anatomy exhibits three levels of logical types of descriptive propositions:

1. The parts of any member of Creatura are to be compared with other parts of the same individual to give first-order connections.
(E.g., Symmetry, Serial Homology)

2. Crabs are to be compared with lobsters or men with horses to find similar relations between parts (i.e., to give second order connections).
(E.g., Phylogenetic Homology)

3. The comparison between crabs and lobsters is to be compared with the comparison between man and horse to provide third order-connections.

We have constructed a ladder of how to think about--about what? Oh, yes, the pattern which connects.
Mind & Nature: A Necessary Unity