As a second example of the derivation of patterns, we may mention _Vipera aspis_, which varies enormously as to its mid-dorsal markings, forming, in different individuals or even on different parts of the body, single or paired spots, a zigzag band, or transverse bars; all these are derived from the paired spots of series D. Each pair of spots may fuse and form transversely oval or elliptical spots or bars, or the spots may assume an alternate disposition from which, through confluence, the zigzag or sinuous band results. Thus, spotted and striped patterns may be traced to a common origin, however fundamental the difference between them appears at first sight. If the elements of the four series, D, DL, L, and VL, unite transversely with each other, and also with the spots on the ventral surface, we obtain ringed forms such as the Coral-snakes. That the black nuchal collar of our common Grass-snake is actually formed by the fusion of the spots of three originally distinct series has been proved by tracing the development of the markings in the embryo. In various species a pair of light streaks extends along the back, bordering the D area, without interfering with the other markings, as we see, among European snakes, in some specimens of _Tropidonotus natrix_ and _viperinus_, and _Vipera berus_. Although it sometimes happens that a definite system of markings prevails throughout a genus, such as the annulate form in the South American _Elaps_, this is far from being universally the case; many closely allied species, or individuals of the same species, may be distinguished by very different patterns. Even on the same individual we may find two opposite types of markings without any transition, as in two Central American species of widely different genera, _Polyodontophis annulatus_ and _Zamenis mexicanus_, in which the anterior part of the body is annulate or barred, and the rest longitudinally striped.