34O MORRIS M. WELLS. 



the streams. Some fishes select certain kinds of bottom for 

 nesting sites, and in doing so probably react to light, temperature 

 and dissolved gases. 



What particular factors are most important in the selection of 

 breeding sites is not at present clear; it will probably be found 

 that many factors are acting in most cases, but that some few 

 factors are of such common importance in the breeding reactions 

 of large groups of fishes that they may be used as an index to the 

 breeding behavior of such groups. 



In connection with the breeding behavior of fishes there are, 

 besides the general reactions involved in the selection of suitable 

 situations for depositing the eggs, a great number of more specific 

 reactions which are often of the utmost importance in the suc- 

 cessful rearing of the new generation. Such reactions are 

 familiar and are illustrated by those of nest-building, aerating the 

 eggs by different devices, guarding the eggs, etc. 1 



2. Resistance of the Eggs and Fry. In spite of the reactions of 

 the adult fishes, which tend to protect the eggs, it is true that of 

 the many thousands that one female may deposit, but very few 

 ever reach the adult stage (Paige, '08) and in most cases the 

 greatest mortality comes at the period when the eggs and fry 

 are developing, (Anthony, R., '08). At this time the relative 

 resistance of the fishes is very low, and many of the eggs probably 

 do not develop because of too great variation in the surrounding 

 conditions; others may develop into abnormal forms which are 

 unable to survive the juvenile period. 



Ransom ('66) has shown that a certain concentration of oxygen 

 is necessary in the development of fish eggs ; he has also shown 

 that carbonic acid arrests development and may result in the 

 production of abnormal forms which never reach the free swim- 

 ming stage. Milner ('72) observed that a supply of oxygen was 

 necessary in the successful shipping of eggs, and Loeb ('12) 

 states again the results of some experiments performed upon the 

 eggs of the sea-urchin, in which he found that these eggs can 

 develop only in the presence of free oxygen, that if the oxygen 

 is withdrawn development stops, but begins again if the oxygen 

 is readmitted. He also states that the process of fertilization 



1 See citations in Shelf ord, "Ecological Succession," III., 'n. 



