Note 31 — Chapter V · THE MORPHOLOGICAL COMPOSITION OF ANIMALS, CONTINUED.

A qualifying fact should be named. When the production of vertebral segments has become constitutionally established, so that there is an innate tendency to form them, there arises a liability to form supernumerary ones; and this, from time to time recurring, may lengthen the series, as in the body of a snake or the neck of a swan. This qualification, however, affects equally the hypothesis of an ideal type and the hypothesis of mechanical genesis.