There is individual variation. This came from observing animal breeding. This observation also came from animal breeders. Through many generations of time, evolution is the result. This sort of classification was based upon an analogy with the human example.
Analogous relationships focus on function while homologous relationships focus on origins.
In biology, we would say that the wings of birds and insects are analogous because they have a common function. However, they are not homologous structures because they do not share a common origin in evolutionary history. A whale flipper, a human hand, and bat or bird wings are homologous structures because they share a common ancestry. Even some contemporaries of Charles Darwin used analogous relationships to classify organisms. Such schemes make an attractive display but tell us nothing about the underlying relationships between organisms.