ADAPTATION OF LAND REPTILES TO LIFE IN WATER 65 



amphibians and primitive reptiles. It is evident that a movable 

 neck of considerable length would not only be of no use to the 

 swiftly swimming fish, but a positive disadvantage to it. The 

 body is quickly and easily turned by the powerful tail fin, and a 

 long neck could be of no use that the tail would not better subserve. 

 It is therefore of interest to learn that, as a rule, aquatic animals 

 of all kinds having a powerful propelling tail have also a short neck, 

 acquired either by the loss of neck vertebrae, or, as in the mammals, 

 by the shortening and coalescence of the normal number of seven. 

 There are very few exceptions to this rule of a short neck and a 

 long tail. Those strange little reptiles of Paleozoic times, the first 

 that we know that returned to the water, the Proganosauria, have 

 not only a long, flattened tail, but also an unduly elongated neck 

 of from nine to twelve vertebrae. 



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 fife, 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 corre- 

 lation 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 serpen- 

 tine movement, like that of a snake upon land, extending through- 

 out 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. 



