264 ME. JULIAN S. HDXLEY ON 
.1 new period is thus introduced in which new objects are introduced to the 
bird. Her excitation, so far as it is determined internally^ remains at the 
same level, but her activities are determined also by the objects presented, 
and she now begins to incubate. In the absence of nest and eggs she would 
revert to the behaviour of the preceding stage — coition and nest-building 
once more, as is shown by the behaviour of birds whose nest and eggs are 
destroyed after incubation has begun; but in the absence of the internal 
stimulation by the secretion of the gonad, she would neither brood, nor be 
eager for coition, nor build a nest. 
The annual cycle of a bird is therefore divisible into a series of periods, 
determined (a) by the state of internal stimulation, {b) by external objects — 
the presence or absence of mate, eggs, young, &c. These latter will 
determine the division of the breeding-period into a number of sub-periods. 
In the third place, (c), an influence on behaviour is exerted by general 
environment — e.g. by the state of the weather, the abundance of food, &c. 
So long, therefore, as the internal stimulation persists at the same intensity,. 
the sub-periods will be mainly determined by the exteruiil objects presented 
to the bird. But, while any one sub-period will be characterized by one 
predominant type of behaviour, the other types or behaviour will be latent. 
in the bird all the time, and may emerge under special circumstances,, 
though rarely, so long as the breeding-season (z. e. the secretion of the gonad), 
continues. 
To return to Selous's observations on the Diver. The male had just fed one 
of the chicks ; he then approached the part of the pool where the female and 
other chick were swimming ; the female also swam towards him. On the 
two birds coming to within a certain distance, both made two short 
flights towards each other over the surface of the water; each time the 
flight ended by the birds " as it were walking on the water, as a Penguin 
walks on the land, bolt upright, with the whole of the white expanse of the 
under surface .... showing." It is like "a Penguin running a few steps, 
and then, for an appreciable period, standing upon the water, the naked legs 
and feet just hidden by it." 'On another occasion, the male of another pair 
was seen to alight on the water on its return from the sea, and to end its 
alightiiig in the following manner : it " footed it, for a little, towards another 
one, presumably its mate, .... maintaining with the help of its wings the 
upright Penguin-like attitude." 
On the tliird occasion, the female had started to give a special note with 
head stretched forward ; she then began advancing towards the male in a 
series of little plunges. This stimulated the male to do the same. Each time 
the cry was given, the neck was arched and held rigid. They eventually met, 
and then quieted down. 
It is interesting to note that none of the actions recorded by Selous are 
identical with those seen by me. This is probably due, at least in part, to. 
