356 POPULAR SCIENCE MONTHLY. 



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and effort not being needed, it was not put forth. As there was 

 food enough in the water, they did not need to go on land. As 

 they did not go on land, they did not use their lungs for breathing, 

 the air sac gradually shrank away, or was used for some other pur- 

 pose, and all the parts of the body became adjusted for life in water, 

 as those of their cousins who left the sea became fitted for the life 

 in air. Being now fishes for good, all the progress since then has 

 made them with each succeeding century more and more decidedly 

 fishy. 



And because they are fishes they are contented to live in little 

 brooks, which would not satisfy you and me at all. But our an- 

 cestors in the early days were more ambitious, and by struggle and 

 effort won what seems to us a larger heritage. 



So it happened one spring when the ice melted out from some 

 little brook that flows down from somebody's hills somewhere to- 

 ward some river that sets toward the Mississippi, the little fishes 

 began to run. 



And first of all came the lampreys, but they hardly count as 

 fishes, for they have yet to learn the first principles of fishiness. A 

 fish is a creature whose arms and legs are developed as fins, having 

 cartilaginous rays spreading out fanlike to form an oar for swim- 

 ming. But the lamprey has no trace of arm or leg, not even a bone 

 or cartilage hidden under the skin. And its ancestors never had 

 any limbs at all, for the earliest lamprey embryo shows no traces 

 of them. If the ancestors ever had limbs, the descendants would 

 never quite forget it. Some little trace would be kept by the cling- 

 ing force of heredity, and at some time or another this rudiment 

 would appear. And the lower jaw they lack too, for that is really 

 another pair of limbs joined together in front as it were, a pair of 

 short hands clasped together and never unlocked. 



But though the lampreys have no limbs and no jaws and are 

 not fishes anyhow, they do not know the difference, and come up 

 the brook in the spring, rushing up the rapids, swirling about in 

 the eddies, just as if they were real fishes and owned the brook 

 themselves. They are long, slender, and slippery, shaped like eels, 

 without any scales and with only a little fin, and that along the back 

 and tail, an outgrowth from the vertebral column. The vertebral 

 column itself is limp and soft, the vertebrae only imperfectly formed 

 and made of soft cartilage. In front the lamprey seems to be cut 

 off short, but if we look carefully we see that the body ends in a 

 round disk of a mouth, and that this disk is beset by rows of sharp 

 teeth. A row of the sharpest of these is placed on the tongue, and 

 two of these are above the gullet, for the tongue to scrape against 

 them. And the rest are all blunt and are scattered over the sur- 



