238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



they are engulfed by the inrush of water into the mouth caused 

 by the sudden opening of the jaws. 



Now let us take the case of the larval swordfish {Istiophorus). 

 In the first stage, it will be noticed the jaws are relatively short, 

 of equal length, and armed with teeth. There is a long, and very 

 low dorsal fin, and a relatively large pectoral fin, and a conspicu- 

 ously large eye. From the gill-cover project two long spines, one 

 just behind the upper segment of the eye, the other and much longer, 

 on a level with the base of the lower jaw. In the second stage the 

 upper jaw has slightly increased in length. The dorsal fin is much 

 larger, the upper opercular spine is relatively smaller, the lower 

 has become more slender, and relatively shorter, while a pair of 

 long filamentous pelvic fins have put in an appearance. The third 

 stage shows approximately the adult stage. The upper jaw has 

 become much longer than the lower, and has assumed its swordlike 

 form, the dorsal fin has risen to a great height ; the upper opercular 

 spine has disappeared, and the lower is greatly reduced, while the 

 teeth have vanished. The eye and the pelvic fins have also de- 

 creased in size. There is no reason to believe that the opercular 

 spines and the pelvic fins are "ancestral characters", but rather that 

 they are adjustments to larval life. But the temporary armature of 

 teeth is probably recapitulatory since they are reduced to a vestigial 

 condition in the adult. 



Animal bodies being, as I contend, "self -regulating", they naturally, 

 and of necessity, move along the old track as they gradually approach 

 the final form attained by the race by adjustments to conditions 

 imposed by changing and changed modes of feeding, for that is what 

 these changes commonly mean in the long run. 



It does not follow that the particular changes in the form of the 

 fins of the angler fish must also be regarded as ancestral adult char- 

 acters. By no means; they are adjustments which have been made 

 by successive larval ancestral stages in adjustment to the particular 

 stimuli set up in these rays by the efforts made to retain their position 

 in mid water and their source of food, just as much as the wood- 

 pecker's tongue, or the three-toed foot, are adjustments to intensive 

 stimuli. Organs, or parts of organs, as I have already urged, change 

 their form, and increase their size — and therefore their efficiency — by 

 use. The first woodpecker probably caught ants as easily as the 

 last of his race. But just as the continuous and peculiar movements 

 of the tongue made by the first ant catchers gave rise to the long, 

 protrusible tongue, so the stimuli given by the formic acid exuded 

 from the bodies of the ants, gradually increased the size of the 

 salivary glands. These larval fishes have changed after the same 

 fashion. Their long fin-rays become reduced as their use declines, 



