326 ANNALS NEW YORK ACADEMY OF SCIENCES 



From a phyletic point of view, on the other hand, the origin of the 

 rapidly changing behavior correlated with morphogenetic changes is less 

 difficult to understand. We have, first of all, numerous grounds for 

 concluding that the larval stages of teleosts are secondary, and that 

 eccentric forms,- — i. e, those with extraordinary fins, colors, outlines, — 

 are derived from "larvae" in which such extreme structures did not exist. 

 "In the case of the eels, therefore, we can reasonably picture a progressive 

 form of development, such, for example, as occurs in many of the older 

 groups of teleosts. We next suggest that within this progressive series 

 of closely similar stages, one stage should, become especially important 

 as adapting the young to a particular environment. The young eel then 

 would tend to remain longer unchanged in the special environment 

 favorable to its feeding, movements, lack of pigment, temperature-re- 

 quirements, etc., and this phase in its life-history would come to sup- 

 plant the adjacent steps in the progressive series. In other words, if we 

 grant that the development of a young eel (the montee) might be accom- 

 plished in the space of fifty weeks, and that at the end of this period it 

 completed the fiftieth of its intergrading stages, we could also admit 

 that with the same total period of larval growth certain of the stages 

 might have expanded while others contracted. Thus, to take an ex- 

 ample, stage twenty, which earlier may have been passed through in a 

 week, might become successively protracted to two weeks, three weeks, 

 or months. And the stages of the montee' intervening between twenty 

 and fifty would be correspondingly reduced. The interval was at first 

 thirty weeks, and included thirty stages; it was next, say, fifteen -weeks 

 in which to represent thirty stages, and finally in the case of our Lepto- 

 cephalus, it was reduced to the astonishingly short term of three weeks, 

 in which to represent the many stages. In such an instance there can 

 be little question that the marked changes in behavior are correlated 

 with abbreviated phases of development. 



