LIFE HISTORY STUDIES OF ANIMALS. 491 



transformations. I liave already discussed this case at greater length 

 than is possible just now.' I have pointed out that the less specialized 

 insect larva*, e. g., those of orthoptera, make a close approach to some 

 wingless adult insects, such as the thysanura, as well as to certain 

 myriopods. Fritz Milller seems to me to be right in saying that the 

 larvte of uonmetamorphic insects come nearer than any winged insect 

 to primitive tracheates. The transformation of the bee, moth, or blow- 

 fly is transacted after the stage in which the normal tracheate structure 

 is attained, and I look uj^on it as a peculiar adult transformation, hav- 

 ing little in common with the transformations of echinoderms, moUusks, 

 or crustaceans. 



In the same way I believe that some amphibia have acquired an 

 adult transformation. Frogs and toads, having already as tadpoles 

 attained the full development of the more primitive amphibia, change 

 to lung-breathing, tailless, land-traversing animals, able to wander 

 from the place of their birth to seek out mates from other families, and 

 to lay eggs in new sites. 



Meduspe furnish a third example of adult transformation which 

 seems to find its explanation in the sedentary habit of the polyp, 

 which probably nearly apx^roaches the primitive adult stage. But 

 here the case is further comj)licated, for the polyp still proceeds from 

 a planula, which is eminently adapted for locomotion, though per- 

 haps within a narrower range. We have two migratory stages in the 

 life history. Each has its own advantages and disadvantages. The 

 planula, from its small size, is less liable to be devoured, or stranded, 

 or dashed to pieces, but it can not travel far; the medusa may cross 

 wide seas, but it is easily captured and is often cast up upon a beach 

 in countless multitudes. 



Adult transformation may be recognized by its occurrence after the 

 normal structure of the group has been acquired, and also by its spe- 

 cial motive, which is egg laying and all that pertains to it; the special 

 motive of larval transformation is dispersal for food. 



The reproduction of the common eel has been a mystery ever since 

 the days of Aristotle, though a small part of the story was made out 

 even in ancient times. It was long ago ascertained that the eel, which 

 seeks its food in rivers, descends to the sea in autumn or early winter, 

 and that it never spawns, nor even becomes mature, in fresh waters. 

 The eels which descend to the sea never return, but young eels or 

 elvers come up from the sea in spring, millions at a time. The elvers 

 have been seen to travel along the bank of a river in a continuous band 

 or eel-rope, which has been known to glide upward for fifteen days 

 together. It was of course concluded that spawning and early devel- 

 opment took place in the sea during the interval between the autumn 

 and spring migration, but no certain information came to hand till 

 1896. Meanwhile this gap in our knowledge was a perplexity, almost a 



1 Nature, December 19, 1895. 



