GROWTH OF SALMON AND TEOUT. 



75 



As a remarkable instance I will mention Carl Semper' a well-known 

 experiments with a species of snails, by means of which he demonstrated 

 that the young of a particular kind of snail, Limnea Stagnalis, even though 

 all other conditions of growth were identical, exhibited a widely differing 

 development which corresponded to the volume of water with which each 

 individual was supplied. When the supply of water exceeded a certain 

 volume this difference ceased, and Semper explained this by the hypothesis 

 that the water must contain some ingredient favourable to the growth of 

 the snail, but that a certain volume of water was essential to ensure that 

 this ingredient was there in sufficient amount. 



It is not impossible that fish may be similarly affected. 



I will also, by way of illustrating another peculiar example of the exist- 

 ence of different influences, 

 describe what I invariably 

 found during my investiga- 

 tions of salmon and trout. 



The reader will perhaps 

 remember that in all fish 

 which have undergone a con- 

 siderable change of growth, 

 this alteration occurs at no 

 particular age, and that the 

 time at which it takes place 

 (the migration age) varies. 

 If, for example, we take fish 

 which have lived for the 

 same number of years after 

 migration and group them according to their migration-age, that is to 

 say, the age when change of growth commenced, and if we compare the 

 development of each of these groups, we shall find one striking fact 

 generally in evidence. The material in my tables is not always large 

 enough for the necessary comparison, but in the one which follows I have 

 selected some examples, and a reference to the tables will in any case show 

 that wherever the size of the material is sufficient this condition of affairs 

 is nearly always to be seen. 



The table, then, indicates that it is a matter of great importance for the 

 growth of a fish subsequent to migration, whether it has migrated at a quite 

 young stage or at a later period of life. The oldest migrants have clearly 

 quite outdistanced the younger ones in growth during the course of the 

 same period of time. They have acquired a far greater weight per individual, 

 which must not be confounded with the difference in weight which might 

 have existed between the oldest and the youngest at the time of migration. 



FIG. 32. Relative size of snails, derived from the 

 same cluster of eggs of the same age, and reared : 

 a. in 100 ccm., b. in 250 ccm., c. in 600 com., 

 and d. in 2000 ccm. of water per individual. 

 (From Semper.) 



