Much advance has been made in the knowledge of the motions of plants as regards their various forms. Growth in plants is not continuous; but is a series of rests and advances. In other words it is rhythmic. The nodes, or knots, in the stems of grasses are resting-places. When a rest occurs, energy may be exerted in a different direction, and a change of form result. This is well illustrated by the common Dogwood of northern woods, _Cornus florida_ on the eastern, and _Cornus Nuttallii_ on the western slope of the American continent. On the approach of winter the leaf is reduced to a bud scale, and then rests. When spring returns these scales resume growth and appear as white bracts. In the annexed illustration the scales that served for winter protection to the buds are seen at the apex of the bracts. In other species of Dogwood the bud scales do not resume growth. Energy is spent in another direction. In this manner we have an insight as to the cause of variation, which was not perceived even so recently as Darwin’s time. We now say that variation results from varying degrees of rhythmic growth—force; and that this again is governed by varying powers of assimilation. [Illustration: OUTLINE OF A WHITE DOGWOOD FLOWER (_Cornus florida_), SHOWING BUD-SCALES DEVELOPED TO BRACTS.] The Darwinian view, that form results from external conditions of which the plant avails itself in a struggle for existence, is still widely accepted as a leading factor in the origin of species. Those which can assume the strongest weapons of defense continue to exist under the changed conditions. The weaker ones do not survive, and we only know of them as fossils. This is termed the doctrine of natural selection.