342 MORRIS M. WELLS. 



dioxide, as was clearly shown in the experiments described in 

 this paper, and that they will turn back from such concentrations 

 has been shown by Shelf ord and Alice ('13) and in some later 

 experiments by myself. With regard to the comparative 

 sensitiveness of young and old fishes, Wiegelt ('85) found that 

 young fishes were more sensitive to ammonia than adults, and 

 Shelford and Alice found the same relation to be generally true 

 for their fishes, in the cases of oxygen and carbon dioxide. Such 

 being the case, in natural lakes and streams fishes would generally 

 find a way out of areas of adverse conditions (Green, '09) if the 

 conditions did not appear too rapidly, and the first to become 

 active in seeking escape would be the young fishes. 



Many complications suggest themselves in a consideration of 

 the ability of fishes to persist in a given environment. One 

 will be considered as suggestive. What will be the result of 

 planting a species of fishes in an environment, in which by its 

 combined resistance and behavior reactions, the species is able 

 to reproduce, but at a great disadvantage, as for instance high 

 egg and fry mortality? 



This question is tied up with the matters of acclimatization, 

 adaptation, etc. There are two main possibilities. First, the 

 adverse conditions may result in the resistance of each suc- 

 ceeding generation being raised; or second, the result may be a 

 lowering of the specific resistance, with each succeeding genera- 

 tion. Lowering the resistance of the species will soon lead to 

 extermination; raising it may result in the species' becoming 

 prolific in waters that at first threatened extermination. 



Whether fishes adjust themselves to conditions is not yet 

 clear. There is evidence that they may adjust, in the fact that 

 many lower forms are able to do so. Wood Jones ('12) states 

 that corals of the same species show great variation in structure 

 in different environments; Moore ('97) states that very young 

 oysters will become acclimated to new conditions when older 

 ones will not, and ('08) he further states that "commercial 

 sponges are very susceptible to the influences of the environment 

 and when transplanted from one place to another speedily 

 change in character." Whether fishes will so adjust is a matter 

 for investigation and especially since, so far as is known, fishes 

 are not particularly plastic at any stage. 



