Gatty Marine Laboratory, St. Andrews. 219 
occur in great numbers at a distance from the tidal region. 
The adolescents, again, have been met with at considerable 
depths. The adults, on the other hand, are found under 
Fuct in tidal rivulets and under stones between tide-marks. 
Though common, they never form groups, but occur singly or 
in pairs, and their numbers (in contrast with those of the 
young) are insignificant. The large number and hardy 
nature of the latter indicate that a great increase would 
annually take place if their ranks were not seriously thinned 
during growth. The migration of the young seawards 
probably increases the chances of survival, though some 
might consider that such augments the danger. Much 
depends, however, upon the eagerness with which the early 
stages are pursued by littoral, laminarian, or pelagic fishes. 
Fifteen-spined Sticklebacks are by no means numerous 
between tide-marks, though at the same time they are gene- 
rally distributed. They occur in rock-pools (especially those 
approaching high-water mark), where they make their nests. 
Their eggs are not so numerous as in the foregoing forms, 
but they are thus specially protected. ‘The young are 
plentiful in the rock-pools in June and present hundreds for 
every adult along the coast-line. As the young seem to keep 
to the rock-pools during growth, it is in the tidal region that 
the decrease in their numbers takes place, and it may be 
that great mortality ensues before the spines are fully 
developed, though much weight need not be attached to this 
feature. Both littoral and pelagic fishes, and, in their early 
stages, Mysidee and other crustaceans, have them for prey. 
The result is that their ranks dwindle to a few adults 
here and there in the rock-pools of a region, their protective 
spines and hard surface having failed to do more for their 
preservation. 
The next species, the Five-bearded Rockling, belongs to 
a different category, since its eggs are numerous and pelagic. 
The adults occur sparingly between tide-marks in rock-pools, 
and seldom seek the adjoining area seawards. The pelagic 
eges tend to scatter the species widely, and the larval and 
post-larval stages follow the same pelagic habit. ‘The young 
are familiar as silvery mackerel-midges in the surface 
tow-nets all round our shores. Thus the eggs and young 
may wander far from the place of their nativity and spread 
the species on new sites; yet neither the large number and 
transparency of the former nor the wide distribution of the 
latter enables the species to increase beyond a certain. 
specified limit, and this though there is no interference by 
man. The contrast between the life-history of this form and 
