646 
SCANDINAVIAN FISHES. 
on the other hand, being confluent with the 2nd (fig. 
162). 1 specimen in 61. 
0 I IV II 0 0 111 
1 — 2 — 3 — 4 — 5 — 6 — 0 — 8 — 9 
i: with a supernumerary spinous ray on the 2nd inter- 
spinal plate, otherwise similar to a. 1 specimen in 61. 
0 IV I II 0 0 III 
1 — 2 — 3 — 4 — 5 — 6 — 0 — 8 — 9 
C: in one Three-spined Stickleback with a rudiment of a fifth 
spinous ray and with eight interspinal plates. 
0 0 I II 0 V IV III 
1 — 2 — 3 — 4 — 5 — 6 — 7 — 8 — 9 
Now, as at least 7 of the interspinal plates — 
though only 3 — 5 at once — may be furnished in the 
Three-spined Stickleback with true spinous rays, and 
as 7 — 12 free spinous rays occur at the dorsal margin 
Fig. 162. Gasterosteus aculeatus , trachurus , tetracanthus ( spmulosus ) 
with the supernumerary spinous ray (*) set on the posterior, de- 
tached part of the third dorsal plate. Natural size. From the Frisches 
Haff, off Konigsberg. After Heincke. 
of the Ten-spined Stickleback, while in the Fifteen- 
spined Stickleback the limits of the variation in this 
respect are 13 and 16, the series, with decreasing num- 
bers, is complete from the last species to that first 
mentioned. Heincke assumes that the development has 
taken this direction, with constantly sinking numbers. 
Thus, where more than 3 free spinous rays occur at 
the dorsal edge of the Three-spined Stickleback, we 
have a retrogression to an older stage of development, 
through which the species has already passed. The 
Fifteen-spined Stickleback, according to this theory, re- 
presents the earliest stage of the development of the 
whole family, whose descent should thus be traced 
from fishes of the Fistularioid family. 
The alterations of development, however, lead us 
to a different conclusion, namely that the Ten-spined 
Stickleback stands nearest the probable origin of the 
family. We have already seen that the elongated form 
which is the chief characteristic of the genus Gastrcea 
— though we meet with it in the youngest forms of 
all the Sticklebacks — especially in the character that 
suggests a transition to the Flute-mouths, namely the 
elongation of the snout, is a character of age, a cha- 
racter which develops during growth from a juvenile 
stage with the typical form of the species perfect in 
other respects, but with the length of the snout only 
slightly greater than in the true Sticklebacks. Even 
during the later (older) stages of growth this general 
rule holds good, for the length of the snout shows 
increase, e. g. in proportion to the postorbital length 
of the head (see the following table), both in the Fifteen- 
spined Stickleback and the true Sticklebacks, but is least 
in the Ten-spined species. Another relation, in which 
the changes of development also follow the same direc- 
tion in all the three species now under consideration, 
gives the same result. If we endeavour to formulate 
an expression for the position of the soft-rayed dorsal 
and the anal fins with respect to each other, we find 
that, as a general rule, the anal spine lies somewhat behind 
the perpendicular from the beginning of the former fin 
— in the Ten-spined Stickleback alone it may excep- 
tionally lie somewhat in front of this line — and that 
the percentages for the distance between the former fin 
and the tip of the snout compared with that between the 
anal spine and the same point, run in all three species 
in inverse proportion from the earlier stages of growth 
to the later. The highest percentages, which are thus 
expressions of the lowest stages of development, occur 
in the Ten-spined Stickleback (see the following table). 
But this is not all. In the relations in which the Fifteen- 
spined Stickleback shows a deviating direction of deve- 
lopment,- it stands nearest to the Ten-spined Stickleback. 
For example, if we compare the length of the soft-rayed 
dorsal fin with that of the head, we find that this fin 
is considerably longer in the true Sticklebacks than in 
Gastrcea , and longest in the Three-spined Stickleback. 
The percentage for this relation also increases, as a ge- 
neral rule, during the growth of the Sticklebacks, but 
decreases with age in Gastrcea (see the following table). 
Average in: 
Gastrcea 
spinacliia. 
• Gasterosteus 
pungiiius. 
Gasterosteus 
aculeatus. 
Length of the body expressed in millimetres.. 
64.9 
132.2 
41.2 
56.3 
55.8 
69.7 
„ ,, „ snout in % of the postorbital length of the head 
96,i 
96.3 
51.2 
56. s 
73.6 
74.8 
Distance between the soft-rayed dorsal fin and the tip of the snout in % of the distance between the anal 
spine and the same point... 
97.2 
95.4 
99.9 
97.8 
90.7 
86 .i 
Length of the base of the soft-rayed dorsal fin in % of the length of the head 
52.6 
49.i 
84.i 
88 .i 
86.2 
95.7 
