6 
But, of all the family, the mackerel is the most fitted for 
rapid propulsion and has the most powerful tail; and this, 
you know, means the greatest power of propulsion, for the 
sole natural propulsive power of every fish lies in its tail. 
I once proved this beyond question, thus :—We stay in 
summer in a house so close to the sea that we are in our 
boat within a minute of our leaving our front door, and we 
have there a pill, or salt water pool, in the rocks, about 
thirty feet long by ten wide by three deep, which is left by 
the tide for about six hours in every tide, and into this pool 
we put the fish which we bring in alive from our trammels 
every morning, and watch them until we want them. 
I have watched an octopus in that pool many times. But 
once I cut off the tail of a fish, a pollock I think, and I 
put it in this pool. At first the fish did not realise its loss, 
and we saw the stump of its tail working, but the other fins 
were, as usual, only balancing the fish. There was no 
progression. After a while the fish stopped working the 
stump of the tail, and lay simply balanced. About an hour 
afterwards I came back to it, and it was slowly progressing by 
using its pectoral fins (those next behind the gills) as oars. 
I had seen all I wanted to know, and had ascertained that 
the tail fin was the fin of propulsion, that the fish had sense 
enough to find out when it had lost it, and reason enough 
to adapt its pectoral fins to a use for which they were never 
intended. I then killed the fish, but my conscience did not, 
nor does it, accuse me of any cruelty towards it. It showed 
no symptoms of pain. Indeed, of all the very many 
thousands of fish that I have seen die, I never saw one show 
symptoms of pain. The nearest approach to it has occurred 
in the crimping of skate immediately on its being taken out 
of the water. The crimping is done by drawing a sharp 
knife in three cuts to the bone, on each side of and parallel 
