On the other hand, certain unrelated reptiles of the past, the dolichosaurs, nothosaurs, and plesiosaurs, with a short non-propelling tail, developed a long neck—sometimes an excessively long one in the plesiosaurs. The turtles, some of which have attained a high adaptation to water life, have invariably a short tail and a freely movable, relatively long neck, a neck which Dr. Hay tells us has increased in length from the beginning of their race by the simple elongation of the vertebrae, as in the giraffe, and never by the addition of vertebrae. We may then account it a rule that swimming animals with a long neck have a short tail, and those with a short tail have a long flexible neck. Even in the plesiosaurs there is some variation of the length of the tail in correlation with the neck. Short-tailed animals must necessarily propel themselves through the water by the aid of their legs, especially the hind legs. If one watches an actively swimming alligator he will observe that the front legs are folded or collapsed by the side of the body, while the hind legs, much bent, are used only slightly in propulsion. The animal swims by a marked sinuous or serpentine movement, like that of a snake upon land, extending throughout the tail and part of the body, at least. An animal propelling itself by its limbs could not move sinuously, and use its legs actively at the same time, and it is probable that the long neck has been evolved compensatorily. With this shortening of the neck and sinuosity of movement there is developed in every case a long trunk as well as a long tail. The trunk becomes more slender and cylindrical, more like that of a snake, with an actual increase of the bones composing it, reaching the great number of forty-three vertebrae in that most sinuous of all water reptiles with legs, _Pleurosaurus_ of the Protorosauria. And the tail, primitively having perhaps sixty or seventy vertebrae, may have as many as one hundred and fifty in the more typical aquatic forms. This elongation of trunk and tail must be of great advantage to the swimming reptile, just as the racing scull is a more perfect type of speedy craft than a flat-bottomed scow. Dr. Woodward has said that the fate of all fishes, if they continue their evolution long enough, is to become eel-like.