350 MOSTLY MAMMALS 
guarded with the same care; the male not unfrequently 
stirring up the eggs with his snout, and often keeping up 
a fan-like movement of his fins for the apparent purpose 
of ensuring a continual change of the water. 
As nest-building fishes are comparatively rare, much 
interest attached to an account in the American Naturalist, 
by Messrs. Young & Cole, of the manner in which the 
brook-lamprey (Lampetra wilder?) makes a structure of this 
nature. It is believed that the males precede the females 
at spawning time and commence nest-building before the 
arrival of the latter. The nest-is made among pebbles, but 
it does not seem that the lampreys follow any definite plan 
in its construction. They affix themselves to such pebbles 
as require removing from the nest, and then endeavour to 
swim straight away with them. In the case of a heavy stone 
two lampreys may join forces, The number of fish in a 
nest may vary from one to thirty or forty; but there are 
generally between three and twenty-five. 
Even when no nest is built, the males of some fishes mount 
guard over the eggs; this being the case with the bow-fin 
(Amia calva), so abundant in the lakes of North America. 
Such are some of the chief instances among amphibians 
and fishes where special arrangements—either of structure 
or of habit—are made for the protection of the eggs and 
young; and although these bear but a small proportion to 
the cases where the latter are left to themselves, yet they 
are sufficient to show that in these respects these two 
groups present peculiarities almost or quite unknown among 
other vertebrates. Why such special arrangements have 
been evolved in these cases, or whether the groups in which 
they occur have any advantage in the struggle for existence 
over their fellows, are questions which, for the present at 
least, must remain unanswered 
