144 Dynamic Theory. 



live organ, it constantly, by its reactions, contributes to the building up 

 of the brain and sense organs ; and the reaction of these again stimu- 

 lates further locomotive development, which finally takes the form of 

 fins. These rather at first serve to give greater steadiness and accuracy 

 to the motion and a better control of the animal lay his brain. As long 

 as the animal remains in the water, a good tail is an index of high 

 standing. But on land it is different. 



When the fish took to living part of the time on land, his fins came 

 into play as feet ; not very good ones at first ; but getting better genera- 

 tion after generation. 



The tail reaches its culmination as to size, among the reptiles whose 

 lives are spent principally in the water. Those living chiefly on land 

 no longer need the tail as a propeller. The limbs increase in impor- 

 tance and in the completeness of their adaptation to the life followed by 

 their owner, and in the same proportion the tail is on the wane. The 

 epitome of his race history is repeated before our eyes .every spring, in 

 the development of the tailless frog through the form of its tailed in- 

 fancy. The larval frog, swimming in the water by means of its tail, 

 and breathing like a fish through its gills, gradually puts forth limbs 

 and develops lungs, while tail and gills' as gradually become suppressed. 

 For a short space he uses gills and lungs, tail and legs ; but soon the 

 gills and tail are eliminated, and lungs and legs remain in full force. 

 On land the tail is of no value as a propeller, and sooner or later takes 

 a back seat. On its way to final extermination it makes itself useful to 

 some tribes in a much reduced capacity. To a monkey its function of 

 holding on is the exact opposite of that which sends the fish through the 

 water like an arrow. The horse, ox, &c. , make a mild use of their 

 much-diminished tails as fly-brushes. 



Those enormous reptiles, the Deinosaurs, which inhabited the land in 

 the Jurassic times, must have found their enormous tails a burden, as 

 any useless appendage always is, and, consequently, until their descend- 

 ants got rid of it they would be hampered in the struggle for existence. 



In a much later age we find the great, sloth-like Megatherium using 

 his massive tail as a third hind leg, the three forming a solid tripod, on 

 which he could sit while he reached for the upper branches of a tree. 

 The early birds, which were only modified reptiles, inherited their heavy 

 tails. The ancient bird of Solenhof en, the Archceopteryx, had a tail con- 

 taining twenty vertebras. The tail of the modern bird, which is a de- 

 scendant of such forms, is now a mere stump, having generally nine 

 vertebrae, three of which are fused into one the coccyx. The ancestor 

 of the present Brachyouran, or short-tailed crab, was the Macrouran, or 

 long-tailed crab. In all mammals, whether it is of any use or not, the 

 tail is, comparatively speaking, a rudiment. In certain breeds of sheep 



