834 



SCANDINAVIAN KISIIES. 



Many other such relations iniglit here l)e cited to 

 prove the statement that ilie rclutlmi between Salmo 

 trutta and Sahno salar is the same as that between young 

 Salmon and old and between the males and the females. 

 'J'o judge by all analogous cases there must be some 

 causal connexion between the dift'erence of species, the 

 difference of age, and the difference of sex. 



We find, however, among the changes of age cer- 

 tain peculiarities which indicate that another factor has 



also asserted its influence in the differentiation of the 

 species. We see something of the kind even among 

 the above averages, for example in the average least 

 depth of the tail: — a comparatively large gap appears 

 in the numerical series, usuallv after the ForcU stage, 

 and this gap is greatest in Sahno salar. The most 

 striking example, however, is perhaps the relation be- 

 tween the height of the anal tin (the length of its 

 longest ray) and the length of the body: 



Now it is a well-known rule that after the Forell 

 stage the Salmons change their habitual place of abode. 

 The Gralax (Salmo trutta) leaves the brooks for meres 

 and large lakes, sometimes directly for the sea; but the 

 Blanklax {S. salar) must necessarily repair at this pe- 

 I'iod to the sea, else its development is arrested, and it 

 persistently retains a greater or less similarity to the 

 Gralax. Thus the wide gap in the averages is probably 

 connected in some way with an alteration in the abode 

 and habits of the Salmons. Here we have assumably a 

 third factor in the differentiation of the species. 



A fourth cause of difference in form undoubt- 

 edly lies, here as among the preceding family, in hy- 

 bridism, of which we have just given instances. 



The relation of the Charrs to the Salmons leads 

 us in most characters to the conclusion that the former 

 should be regarded as more advanced stages of deve- 

 lopment of the Salmon type. To exemplify this, we 

 shall only refer the reader to the above-mentioned 

 characters, which afford the following averages: 



,S16 Salmon. 108 Charr. 



Length of the boily exjiressed in luilliiuelres 



., ., head in % of the length of the body 



Postabdominal length „ „ „ ,, „ „ „ „ 



Length of the maxillaries ,, ,, ,, ,, ., ., „ ,, 



Least depth of the tail • „ „ „ „ ,, „ „ ,, 



Length of the middle eaudal rays „ „ „ „ „ „ „ 



Height of the anal Hn „ „ „ „ „ ,, ., „ 



Least depth of the tail „ ,, „ ,, preabdominal length 



In every case whei'e the jiercentage decreases dur- 

 ing growth in the Salmons, it is less in the Charrs, 

 and vice versa. This applies also to an internal cha- 

 racter which usually possesses great validity, the num- 

 ber of the gill-rakers. In the outer row on the front 

 of the first branchial arch Salmo trutta has on an 

 average 14-10, Stdmo stdar 17 — 20, Salimi unilila 

 •23 or 24. 



Hence it is clear that, in the great majority ot 

 cases, the Blanklax must come nearer the Charrs; but 

 that the latter have not been developed directly from 

 the former, appears from other relations, in which they 

 stand nearer the Gralax. One of these relations is 

 afforded by the character which, to the best of our 

 knowledge, gives the only tenable distinction between 

 the Salmons and the Charrs, namely the size of the 



