Organs that are now of little importance may have been crucial in early ancestors and, after being slowly perfected at an earlier stage, have been passed down with minimal changes, even though they are now of slight use. Any deviations that are actually harmful will have been weeded out by natural selection. The tail, which is a crucial organ of locomotion for most aquatic animals, can be found in many land animals, despite their modified swim-bladders or lungs, hinting at an aquatic origin. Given its initial importance as an aquatic organ, the tail might have been co-opted for various purposes, such as a fly-flapper or an aid in turning, as seen in dogs, though the latter use is minimal since hares can still change direction swiftly without a tail.

Secondly, we might overestimate the significance of characters that are actually of little importance, arising from secondary causes unrelated to natural selection. Climate, food, and other factors can have a minor direct influence on an organism's structure; characters can reappear due to the law of reversion; correlation of growth can significantly modify structures; and sexual selection can greatly influence external features to give one male an advantage over others or to attract females. Moreover, a modification that initially arose from unknown causes might not have provided an immediate benefit to the species but could have been advantageous to its descendants under new conditions and with altered habits.
