Analogy and homology
The last form of similarity is that of analogy and homology. Here the similarity is determined by function.
Theories of origin and evolution
A good way to achieve understanding of something is to ask where it comes from and how it came to be what it is: origin and evolution. This is a historical approach where things completely dissimilar can suddenly be shown to have a connection in some remote past. That previously unsuspected connection then changes our view of the objects under analysis.
Homology describes the similarity between, for example, a wing and an arm. Their functional relationship is established on the basis of time. They have a relation because they both developed from the same earlier prototype.
Analogy describes a functional relationship not in time but in space, a similarity whereby, for example, a suspension bridge and the skeleton and muscular system of a jaguar are similar. At a certain level they use similar principles to perform a similar function.
This is different to the similarities conjured up by the Simile and the Metaphor. The simile is simply any kind of comparison: “Her eyes burnt like fire.” That is a simile. A metaphor is a figure of speech, which suggests comparison. In this way “the ship in a stormy sea” can be a metaphor for “the journey of life”. The analogy is a comparison, which is specifically to do with the function of a thing.
Similarities and differences are worth playing with. They help the design process. The liven up inquiry, lift it out of the restrictions of habit.