Passing now to the obscurations in the animal world caused by alterations favouring locomotion, we note first that the locomotive power is at the outset very slight. Among many orders of _Protozoa_, as also among many low types of _Metazoa_, vibratile cilia are the most general agents of locomotion--necessarily feeble locomotion. Regarded in the mass, the _Cœlenterata_, when not stationary like the _Hydra_ or higher types in the hydroid stage, usually possess only such small self-mobility as the slow rhythmical contractions of their umbrella-disks effect, or else such as is effected by bands of cilia or of vibratile plates, as in the _Beroe_. Even among these low tpes of _Metazoa_, however, in which ordinarily the radial structure is conspicuous, or but slightly obscured by an ovoid form as in the _Ctenophora_, we find, in the _Cestus veneris_, extreme obscuration caused by an elongation which facilitates movement through the water; alike by the actions of its vibratile plates and by its undulations, which simulate those of sundry higher animals. And here we come upon the essential fact to be recognized. Elongation favours locomotion in various ways that are severally taken advantage of by different types of creatures. (1) To a given mass of moving matter the resistance of the medium decreases along with decrease in the area of its transverse section, and this implies increase of length: a given force will move the lengthened mass along with greater facility. (2) Reaching a certain point the elongated form enables an animal to progress by undulations, as in the water fish do, and even some cœlenterates and turbellarians do, and as on land snakes do: lateral resistances serving in either case as fulcra. (3) Lengthening of the body serves otherwise to aid locomotion in the creeping or burrowing worm, which, utilizing the statical resistance of its hinder part thrusts onwards its fore part, and then, holding fast its fore part by the aid of minute _setæ_, draws the hinder part after it. But elongation, doubly advantageous at first, while the body is itself the chief instrument of locomotion, gradually loses its advantageousness as special instruments of locomotion are developed. (4) This we see in that locomotive action effected by limbs, which, many and small in the lower _Arthropoda_ and becoming few and larger in the higher, at length give great activity to a shortened and consolidated body: a stage reached only through stages of decreasing elongation accompanying increase of limb-power. (5) In the _Vertebrata_ locomotion by undulations comes, along certain lines of evolution, to be replaced by that limb locomotion which accompanies the rise from water-life to land-life: the evolution of Amphibians exhibiting the transition. (6) Further, we see among mammals that as limbs become efficient the elongated body ceases to be itself instrumental in locomotion, but that still some elongation remains a characteristic. (7) Finally, where