234 ANIMAL SKETCHES. CHAP. xvi. 



appears to be a direct continuation of the body, and has a 

 stout rod of gristle, continuous with the backbone, run- 

 ning along its upper margin. From this upper portion 

 there hangs the lower lobe of the tail, like a rounded cur- 

 tain, being supported on delicate fin-rays which fan out 

 from two plates near the base of the rod of gristle. The 

 tail is, in fact, as unsymmetrical as that of a little dog-fish. 

 And this is a most interesting fact. For the dog-fish is in 

 this matter on a lower level of fish-life than the stickle- 

 back, just as the tailed newt is on a lower level of 

 amphibian life than the tailless frog. And just as the 

 frog in its juvenile tadpole condition passes through a 

 tailed newt-like stage, so does the little stickleback pass 

 through a stage in which it so far resembles the less 

 differentiated dog-fish. Both are illustrations of the 

 biological law or fact, that individual development is a 

 more or less condensed epitome of race-development. 



As the little stickleback increases in size its tail becomes 

 more and more symmetrical in shape, and the fish takes 

 on more completely the form of its parents. The lillipu- 

 tian Thornie that now has the sole possession of one of my 

 tanks, and is, as I write, engaged in darting after and 

 devouring with avidity minute water-fleas, is some of 

 an inch long, having attained that size in about two 

 months. His sides are silvery white, with dark vertical 

 bands. He is the sole-survivor of the brood which my 

 rosy-gilled Thornie, now quite sedate and sober-hucd, for 

 the courting season is over, hatched out in his horse-hair 

 nest. 



