556 MR. J. S. HUXLEY ON THE 
crepuscular would be more accurate, especially for the Nightjar, 
who is silent for at least the three or four midnight hours); 
while in some species, like the Redshank (or the cock Nightingale 
in spring), periods of activity and rest alternate throughout the 
twenty-four hours. 
4. Rest and reserve. 
In spite of its being active only during the daylight hours, the 
Grebe spends many of these resting, in the attitude so well 
described by Selous as resembling a pork-pie (see Pl. I. fig. 2). 
This is but one further instance of the principle of reserve that 
runs through all life. In watching birds two forms of this are 
especially brought under notice—the reserve of time and the 
reserve of nervous energy that are present in normally favour- 
able conditions. 
In the Grebe, the many hours of the day spent in sleeping, 
or at least in what Sidis calls the hypnoidal state, represent 
the time-reserve. These extra hours of sleep, of course, in- 
crease the energy-reserve. This latter is,in most birds, got rid of 
in actions which seem entirely without biological significance— 
they merely excite pleasure by releasing the energy in bodily 
movement or in sound: think of the pleasure-flights of gulls in 
early spring, or of swallow-broods in late summer (here accom- 
panied by twitterings), or the antics of wagtails in fine autumns 
on the lawns. In the spring, however, the surplus energy of 
many birds has been seized upon by Sexual Selection, and used 
up in fighting or in display (ef. Wallace's general ideas on the 
role of energy in Sexual Selection, and Howard (713)).) 
the Grebe, similarly, it has been diverted into fresh channels 
through Mutual Selection, and thus pressed more directly into 
the service of the species. 
5. Powers of learning by experience. 
As an interesting side-light on the psychology of these birds, I 
will record an incident seen by my brother. 
A Grebe had caughta very large fish and was trying to swallow 
it in the usual way, first throwing its head violently back and 
then stopping it suddenly, thus jerking the fish (which, of course, 
is held so that its head was foremost) down the throat. This fish 
however, was too big. After a long period of fruitless jerking, 
the bird was forced to put the fish out into the water. The fish, 
being still alive, swam off. This was too much for the Grebe, 
who at once dived, caught it again, and again attempted i 
swallow it. Naturally it had again to put it out and the whole 
process was repeated. It attempted to swallow the fish four times : 
the fifth time it let it swim off as best it could. The whole thing 
is thus a reflex chain: ‘“ See fish—catch fish—try to swallows 
use—put fish out:—See fish....” and soon. It did, however 
profit by experience, for each time it made rather fewer efforts th 
swallow it, and at last stopped its fruitless trying altogether, 
