120 ESSAYS OF A BIOLOGIST 



watched their habits with any thoroughness would 

 agree. It also acts, by a simple extension of func- 

 tion, as an informative symbol. The other bird 

 knows what is meant; it raises its head from beneath 

 its wing, gives a sleepy, barely discernible shake — and 

 replaces the head. In so doing it puts back the possi- 

 bility of the ceremony and the thrill into its slum- 

 bers; for it takes two to make love, for Grebe as for 

 human. The cock swims off; but he has a restless 

 air, and in a minute or so is back again, and the same 

 series of events is run through. This may be re- 

 peated three or four times. 



If now another hen bird, unaccompanied by a 

 mate, reveals herself to the eye of the restless and 

 disappointed cock, he will make for her and try the 

 same insinuating informative head-shake on her; 

 and, in the cases that I have seen, she has responded, 

 and a bout of shaking has begun. Flirtation — illicit 

 love, if you will; for the Grebe, during each breed- 

 ing season at least, is strictly monogamous, and the 

 whole economics of its family life, if I may use the 

 expression, are based on the co-operation of male and 

 female in incubation and the feeding and care of the 

 young. On the other hand, how natural and how 

 human! and how harmless — for there is no evidence 

 that the pretty thrills of the head-shaking display 

 ever lead on to anything more serious. 



But now observe. Every time that I have seen 

 such a flirtation start, it has always been interrupted. 

 The mate, so sleepy before, yet must have had one 



