PERCENTILE PROPORTIONS OF LAKE AND ATLANTIC SALMONS. 123 



length of the fish is increased, but it is a seasonal increase which subsequently may be 

 more or less reduced. 



During the growth of the fish certain proportions are increased in direct ratio to its 

 length, until they are modified by other factors. Certain other proportions are reduced 

 in indirect ratio to the length of the fish, up to a point when they, too, are subjected to 

 modifications. Still other proportions change very httle or not at all from parr to adult. 

 Some parts of the fish grow faster than other parts, with consequent increase and de- 

 crease in relative proportions. Those structures which develop pari passu with the in- 

 crease in length of the fish retain their relative proportions unchanged. 



It seems reasonable to beUeve that forms which differ in the way that some of the 

 variations are constantly manifested, also differ physiologically. Such differences are 

 phylogenetic and constitute what are here regarded as 'Natural Species' which are often 

 confused with the taxonomic subspecies. 



Most of the characters employed by ichthyologists in the classification of salmonids 

 pertain to external countable and measurable structures and dimensions. Occasionally 

 certain internal structures are considered. The external countable structures usually 

 receiving attention comprise fin rays, gill rays (branchiostegals) , and series of scales. 

 The internal structures most commonly noticed are the numbers of gill rakers, vertebrae, 

 and pyloric coeca. 



While these characters vary to some extent they do not change with the growth of the 

 fish or under other conditions before mentioned. For this reason they have been re- 

 garded as some of the most dependable of structural characters and almost always have 

 been used in descriptions. 



Measurable characters comprise certain dimensions in relation to each other. These 

 are the characters which give the fish its shape. Some ichthyologists indicate the rela- 

 tion of one dimension to another by stating the number of times the one structure 

 (meaning its dimension) is contained in another. For example, the head is said to be 

 contained so many times in the length of the fish; or, the eye, so many times in the head. 

 Others employ percentile proportions, to indicate this relationship of structures. Thus 

 instead of sajdng that the head is contained 4 times in the body, it is stated that the 

 length of the head is 25 per cent of the length of the body. 



This is the method used in tables 21 and 22, which respectively compare the per- 

 centile proportions of non-breeding and breeding salmon. While the number of in- 

 dividuals entering into the comparison are too few and the range of sizes too hmited to 

 afford positive conclusions, they serve to indicate, in a measure, the trend of variations; 

 whether they are wide or narrow, and approximately what structures and dimensions 

 vary the most. 



The tables show the averages of 25 percentile proportions (fig. 1) arranged in ascend- 

 ing scale according to the average total lengths of eight groups each contaming a few 

 individuals of both sexes. The averages of percentile proportions should indicate in a 

 measure, to what factor, — whether age, size, sex, or breeding condition, — these 

 variations are attributable, even though the number of fish in each group is small. In 



