54O MR. J. S. HUXLEY ON THE 
perfectly innocent “Y.” The former, from my records, seems to be 
more usual (as one would expect: it is more probable that a lone 
bird will be more eager to “shake” than one whose mate is near 
at hand). 
One interesting fact emerges from table A (p. 542): in seven 
out of eight cases where there was a flirtation and I could be sure 
of the sexes, “ Y” was a female, while of the cases where X was 
indifferent or hostile to “ Y,” ““ Y” was female in about half the 
number. 
This is probably not merely chance; I believe that here the 
male Grebe possesses a little more of the normal characteristics 
of males. 
I have several records where “ Y” is very threatening from 
the first; here the pair always seem to make common cause 
against the intruder. I do not see how these cases can have 
anything to do with a desire of “ Y” to shake, ete., nor is there in 
the behaviour of the pair any sign of jealousy. ‘There seems to 
be only hostility, and [am disposed to think that in all such 
eases the pair has been trespassing on another's territory 
(see p. 558). 
Y may drive “ Y” right off, or content itself with going into 
the threatening attitude. ‘“Y” is always driven off if Y sees a 
flirtation going on; but when the pair are simply swimming past 
the odd bird, Y contents itself with going into the threatening 
attitude (in one doubtful case only was “ Y” driven away) ; 
here we have association at work. In cases where “ Y” hangs 
about, it may be driven off by Y even though no flirtation takes 
lace. 
If Y wishes to drive “ Y” off, it usually dives. Sometimes 
““Y” is completely taken by surprise, and, as it flies off, its place 
is at once taken by the jealous one. Presumably Y’s beak 
sometimes actually comes into play, which must be very. un- 
pleasant for ‘“Y.” At other times “‘ Y” sees Y dive, and is off 
at once. 
The mere presence of an odd bird is not sufficient stimulus to 
induce a bout of shaking between the pair. Shaking is usual 
(though not invariable) after a flirtation and subsequent driving 
away of ‘ Y.” 
When a bout of shaking does take place after flirtation, it 
always seems to begin in the forward position. This position 
in its full development I have never seen except after an odd 
bird has been driven away, so that we have here an interesting 
example of a definite form of courtship-action used exclusively 
under the influence of jealousy. 
Let us close with one or two interesting cases—actual hap- 
penings. Here is one:— 
1. The cock of a pair, saw an odd bird (? sex) near by, and 
drove it off, by flying at it, spluttering along the water. He 
then came back towards his mate, and from his attitude I thought 
