274 MR. JULIAN S. HUXLEY ON 
the male. But once he has occupied the territory, it will be again a biological 
disadvantage for the female to put off too long her quest for territory-plus- 
mate, or else she too will find herself forestalled. 
On the other hand, the exigencies of the food-supply demand that ovi- 
position should not occur until a certain date, vai'ying naturally for different 
species, but almost always considerably later than the date at which territory 
is occupied. Thus we have two opposed advantages — one in the early 
occupation of territory, one in the late starting of coition. Jfs a result, there 
will be a period after both birds are on the territory — i.?. after pairing-up 
has occurred — when it will be biologically undesirable that oviposition should 
occur. This we may call, if we choose to adopt a not wholly suitable human 
metaphor, the " engagement period." Extremely little is known concerning 
this period in species with mutual courtship : in many of them it is absent, 
the birds only returning to the breeding-grounds, or only pairing up, 
immediately before coition can and does occur. (This is apparently the case 
in Snowy Egret, Louisiana Heron, and many cliff-breeding birds.) In any 
case, what is known is so slight as to make discussion unprofitable. 
In many dimorphic species, on the other hand, a good deal is known. 
Such an "engagement period" occurs apparently in all small Pa sseres which 
have nidicolous and insectivorous young, and which therefore, as Howard 
has demonstrated, imperatively require a territory which is not merely a 
nesting-area, but a hunting-ground from which the needs of the new-hatched 
young may be supplied. As we shall see later, the dimorphism itself has 
probably been evolved in relation to other biological needs of the species — 
viz. the necessity for concealing coloration and behaviour in the female. 
Here, however, we shall take the dimorphism as given. 
The simplest way of satisfying these biological requirements is to make the 
inale enter in early spring on to a sexually-excited state in which he is 
impelled to seek and occupy territory, to pair or attempt to pair with any 
female who stays in the territory, to assist in feeding young when hatched, 
etc. So far as his endocrine excitation goes, lie remains in the same phase 
throughout the season. 
With the female, however, the case is different. She must pass through 
at least two phases of excitation— the first sufficient to impel her to abandon 
the "neutral" non-sexual existence of the winter and to seek and if neces- 
sary fight for (PI. 15. fig. 7) the company of a particular male, but not sufficient 
for coition to take place ; the second, more intense, impelling to coition and 
nest-building ; when the eggs are laid, to incubation ; and, later still, to the 
care of the young. 
There will thus exist a period in which the male will be anxious for coition, 
bnt the female will not permit it. Further, the biological causes for the 
existence of such a period are not epigamic, not concerned with the relation 
of the sexes, but are to be sought in relation to the need for occupying 
