§ 241. Another trait which has to be noticed under this head, is the spiral, or rather the helical, arrangement of parts. The successive nodes of a phænogam habitually bear their appendages in ways implying more or less twist in the substance of the axis; and in climbing plants the twist is such as to produce a corkscrew shape. This structure is ascribable to differences of interstitial nutrition. Take a shoot which is growing vertically. It is clear that if the molecules are added with perfect equality on all sides, there will be no tendency towards any kind of lateral deviation; and the successively-produced parts will be perpendicularly over one another. But any inequality in the rate of growth on the different sides of the shoot, will destroy this straightness in the lines of growth. If the greatest and least rates of molecular increase happen to be on opposite sides, the shoot must assume a curve of single curvature; but in every other case of unequal molecular increase, a curve of double curvature must result. Now it is a corollary from the instability of the homogeneous, that the rates of growth on all sides of a shoot can never be exactly alike; and it is also to be inferred from the same general law, that the greatest and least rates of growth will not occur on exactly opposite sides of the shoot, at the same time that equal rates of growth are preserved by the two other sides. Hence, there must almost inevitably arise more or less of twist; and the appendages of the internodes will so be prevented from occurring perpendicularly one over another. A deviation of this kind, necessarily initiated by physical causes in conformity with the general laws of evolution, is likely to be made regular and decided by natural selection. For under ordinary circumstances, a plant profits by having its axis so twisted as to bring the appended leaves into positions which prevent them from shading one another. And, manifestly, modifications in the forms, sizes, and insertions of the leaves, may, under the same agency, lead to adapted modifications of the twist. We must therefore ascribe this common characteristic of phænogams, primarily to local differences of nutrition, and secondarily to survival of the fittest. It is proper to add that there are some Monocotyledons, as _Ravenala madagascariensis_, in which this character does not occur. What conditions of existence they are that here hold this natural tendency in check, it is not easy to see.[42] CHAPTER XIII. MORPHOLOGICAL DIFFERENTIATION IN ANIMALS.