50 /. H. Powers 



walking, too, such individuals do not use the limb after the fash- 

 ion of the half swimming stroke, described above, but the foot is 

 carried almost directly forward, parallel to the body, and in the 

 back stroke the inner toes even pass under the side of the body 

 and of the hip girdle. The inference is natural that this more 

 perfect use of the limb is a question of age in the adult, the adap- 

 tation being to terrestrial rather than to aqu"atic life, more espe- 

 cially an adaptation of the posterior limbs to vigorous action 

 within a narrow burrow. This may be, to some extent, the case, 

 for I find that the limbs of the adult are capable of fairly marked 

 modifications of this nature after metamorphosis has taken place, 

 provided that their situations are sufficiently peculiar, are uniform 

 for the individual, are sufiiciently prolonged, and that the animal 

 is so fed that growth (the chief and first condition of all modi- 

 fication) takes place. In many cases, however, no 'such modifi- 

 cation during adult life occurs. At the present writing, I am 

 photographing a number of animals, known to me as larvae three 

 years ago, and it is surprising to what extent even delicate fea- 

 tures of larval limb-habit have been retained. In any case, the 

 modifications are most striking during larval life. This is true 

 whether we take single instances, of extreme attitudinal and 

 functional difiference, or whether we notice the regular recur- 

 rence of minor dififerences in the case of whole groups of larvae 

 developed under closely related yet somewhat different conditions. 

 As an instance of the latter I may cite again the case of the 

 two groups of larvae developing during the same season, in the 

 same pond, upon the same food, but the larger number of which 

 maintained the free-swimming habit while a considerable but 

 smaller number rested vipon or crawled about the bottom. A 

 series of both of these types was photographed by placing the 

 chloretoned animals on a glass plate under water. When the 

 limbs were thrown forward they would sweep back to a natural 

 position at about a right angle to the body, and as it happened 

 with perfect regularity the feet of the more slender, free-swim- 

 ming larvae would remain almost vertical, refusing to take any 

 other position save as the result of much manipulation, while the 

 feet of the robust bottom-crawlers would naturally rest at an 



246 



