Species, polymorphic, [46].
——, common, variable, [53].
—— in large genera variable, [54].
——, groups of, suddenly appearing, [302], [307].
—— beneath Silurian formations, [307].
—— successively appearing, [312].
—— changing simultaneously throughout the world, [322].
Spencer, Lord, on increase in size of cattle, [35].
Sphex, parasitic, [218].
Spiders, development of, [442].