172 BIRD LIFE GLIMPSES 



just these actions that most resemble those which 

 seem so purposive, in the ordinary building of a 

 nest. All the others seem to me to belong to 

 that large and important class of avine movements, 

 which may be called the sexually ecstatic, or love- 

 mad, group. Nor can these two classes of actions 

 be separated from each other. The motion by 

 which the hollow is produced is accompanied by — if 

 it may not rather be said to be a part of — that most 

 pronounced, peculiar, and, as it seems to me, purely 

 sexual one of the tail, or rather of the anal parts ; 

 and there is, moreover, the very marked and dis- 

 tinctive run, with the set, rigid attitude — that 

 salient feature of a bird's nuptial antics — which 

 immediately precedes the rolling, in the same way 

 that the run precedes the jump in athletics. All 

 this set of actions must be looked upon as so many 

 parts of one and the same whole thing, and to ex- 

 plain such whole thing we must call in some cause 

 which will equally account for all its parts. The 

 deliberate intention of making a nest will not do 

 this, for many of the actions noted do not in the 

 least further such a plan. On the other hand, 

 sexual excitement may just as well produce rolling 

 on the ground — as, indeed, it does in some other 

 birds — and, perhaps, even pecking round about on 

 it, as it may the stiff, set run, and those other 

 peculiar movements. And if some of many move- 

 ments, the cause of all of which is sexual excite- 

 ment, should be of such a nature as that, out of 

 them, good might accrue to the species, why should 

 not natural selection seize hold of these, and gradu- 

 ally shape them, making them, at last, through the 



