The Descent of Man

the seashore must be greatly affected by the
tides; animals living either about the MEAN
high-water mark, or about the MEAN lowwater
mark, pass through a complete cycle of
tidal changes in a fortnight. Consequently, their
food supply will undergo marked changes
week by week. The vital functions of such animals,
living under these conditions for many
generations, can hardly fail to run their course
in regular weekly periods. Now it is a mysterious
fact that in the higher and now terrestrial
Vertebrata, as well as in other classes, many
normal and abnormal processes have one or
more whole weeks as their periods; this would
be rendered intelligible if the Vertebrata are
descended from an animal allied to the existing
tidal Ascidians. Many instances of such periodic
processes might be given, as the gestation of
mammals, the duration of fevers, etc. The hatching
of eggs affords also a good example, for,
according to Mr. Bartlett ('Land and Water,'
Jan. 7, 1871), the eggs of the pigeon are hatched