Viewed by itself, the loss of wings is a degeneration, but in these special conditions of life it is advantageous to the organism in the struggle for life. Among other rudimentary organs I may here, by way of example, further mention the lungs of serpents and serpent-like lizards. All vertebrate animals possessing lungs, such as amphibious animals, reptiles, birds, and mammals, have a pair of lungs, a right and a left one. But in cases where the body is exceedingly thin and elongated, as in serpents and serpent-like lizards, there is no room for the one lung by the side of the other, and it is an evident advantage to the mechanism of respiration if only one lung is developed. A single large lung here accomplishes more than two small ones side by side would do; and consequently, in these animals, we invariably find only the right or only the left lung fully developed. The other is completely aborted, although existing as a useless rudiment. In like manner, in all birds the right ovary is aborted and without function; only the left one is developed, and yields all the eggs. I mentioned in the first chapter that man also possesses such useless and superfluous rudimentary organs, and I specified as such the muscles which move the ears. Another of them is the rudiment of the tail which man possesses in his 3—5 tail vertebræ, and which, in the human embryo, stands out prominently during the first two months of its development (compare Plates II. and III.). It afterwards becomes completely hidden. The rudimentary little tail of man is an irrefutable proof of the fact that he is descended from tailed ancestors. In woman the tail is generally by one vertebra longer than in man. There still exist rudimentary muscles in the human tail which formerly moved it.