26 The Lije Story of the Fish 



never varies. It remains that which he fixed for himself when 

 he first put on scales. It is the same, within close limits, as is 

 worn by all his species. The number does not change, but 

 each individual scale does. Each scale grows to keep up with 

 that fraction of the fish's body which it covered in the be- 

 ginning. And this growth is accomplished by adding rings of 

 new material around the edge, in somewhat the way in 

 which a tree adds to its girth. The new material being fur- 

 nished by the dermis, the after-end of the scale, covered 

 only by the epidermis, shows irregular and poorly marked 

 accretions, but the larger portion which is surrounded by 

 the dermis carries clearly defined rings throughout the life 

 of the fish. 



The tree, as we have all been taught, adds only one ring 

 each year, and it is by counting the total number of rings 

 that the tree's age can be told. Each fish-scale adds many 

 rings each year; and yet the character of this ring-growth 

 varies with the seasons to such an extent that it is possible 

 for a practiced person to look at a salmon scale under a 

 microscope and tell not only how old the fish is, but also how 

 many years it spent in the sea, and how many times it has 

 spawned — even, if it has not spawned, to calculate how long 

 it was at the end of each year of its life. No other animal 

 known to man carries about with it such a complete auto- 

 biography. 



The method by which this autobiography can be read was 

 originated by Johnston and Dahl, working separately in 

 England and in Norway, at the turn of the century, and in 

 the light of their findings, the whole thing seems as simple 

 as Columbus' egg. In summer, when the water is warm and 

 food is plentiful, the fish feeds actively and grows fast. The 

 scale, during this period, must also grow fast in order to 

 keep on covering its portion of the fish, and the growth-rings 

 of the scales are therefore far apart. In winter, on the other 



