The reason for this extreme degeneration is probably to be found in the sandy nature of the soil in which the creature burrows, a substance which would evidently irritate and inflame any functional remnant of an eye.
The gonophores of different hydroids differ greatly in structure from one another, and form a series showing degeneration of the medusa-individual, which is gradually stripped, as it were, of its characteristic features of medusan organization and finally reduced to the simplest structure.
A very early stage in the degeneration is well exemplified by the so-called " meconidium " of Gonothyraea (fig.
Medusa-buds) in the extreme of degeneration, then it follows from Brooks's theory that Hydra must be descended from an archaic form in which the medusan type of organization had not yet been evolved.
Just as in the Mediterranean region, the degeneration of forests has given rise to mhquis and garigues, so in western Europe, the degeneration of forests has brought about different types ot grassland, heaths, and moors.