834 
SCANDINAVIAN FISHES. 
Many other such relations might here be cited to 
prove the statement that the relation between Salmo 
trutta and Salmo solar is the same as that between young 
Salmon and old and between the males and the females. 
To judge by all analogous cases there must be some 
causal connexion between the difference 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, usually after the Forell stage, 
and this gap is greatest in Salmo solar. The most 
striking example, however, is perhaps the relation be- 
tween the height of the anal fin (the length of its 
longest ray) and the length of the body: 
Average 
Fry. 
Parr. 
Forell. 
Oring. 
Lax. 
Length of the body expressed in millimetres Salmo trutta 
47 
108 
182 
313 
587 
,, ,, ,, „ ,, ,, ,, Salmo salar 
50 
109 
161 
403 
609 
Height of the anal fin in % of the length of the body Salmo trutta 
13.c 
13.2 
13.i 
12.i 
11.5 
„ » „ „ „ „ V „ ), „ „ „ ?) - Salmo salar 
12.9 
11.8 
11.2 
8.7 
9.0 
Now it is a well-known rule that after the Forell 
stage the Salmons change their habitual place of abode. 
The Grdlax ( 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- 
riod to the sea, else its development is arrested, and it 
persistently retains a greater or less similarity to the 
Grdlax. 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: 
Avers 
g e 
316 Salmon. 
108 Charr. 
Length of the body expressed in millimetres 
279 
333 
v head 

. in 
% of 
the length of the body 
22.5 
21.5 
Postabdominal length 
- 55 
55 55 
55 55 55 55 55 
19.9 
20.9 
Length of the maxillaries 
- 55 
5 5 55 
55 55 55 55 55 
8.8 
8.5 
Least depth of the tail 
55 55 
55 55 55 55 55 
8.8 
7.2 
Length of the middle caudal rays 
55 55 
55 55 55 55 55 
7.9 
6.9 
Height of the anal fin 
55 55 
55 55 55 55 55 
12.3 
10.5 
Least depth of the tail 
55 55 
,, preabdominal length 
30.7 
24.9 
In every case where the percentage 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—16, Salmo salar 17—20, Salmo umbla 
23 or 24. 
Hence it is clear that, in the great majority of 
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 Grdlax. 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 
