136 THE BIOLOGY OF THE SEASONS 



is full of difficulties, and though birds have been so much 

 studied, there is still great uncertainty as to the significance 

 of their behaviour. It seems as though this must remain 

 until ingenious experiments are devised to test the inferences 

 from observation. 



Every one recognises that the advent of the pairing season 

 is marked by a variety of unusual activities on the part of 

 male birds activities of song and flight, of dance and dis- 

 play, which seem to express excited states of feeling and 

 to prompt similar responses on the female's part. We may 

 sum up the characteristic activities in the word " court- 

 ship," though there are few data which would enable us to 

 decide how far there is deliberate wooing on the one hand or 

 deliberate choice on the other. 



SEX-DIMORPHISM 



No one without a microscope can tell a male sea-urchin 

 from a female sea-urchin, and in the lower reaches of 

 the animal kingdom an external uniformity of the two sexes 

 is very common. As we ascend the series, however, differ- 

 ences between the sexes become more and more frequent and 

 conspicuous. The essential functions of the males and the 

 females become more and more different, as we may see if 

 we contrast starfish with fish, or fish with mammal ; their 

 habits of life diverge ; and to the primary differences there 

 are added all manner of secondary peculiarities. In the 

 higher reaches of the animal kingdom we come face to face 

 with marked " sex-dimorphism," familiar in the contrasts 

 of peacock and peahen, ruff and reeve, stag and hind, lion 

 and lioness. In the contrast of man and woman the 

 dimorphism finds its highest and subtlest expression. 



As to the biological significance of this sex-dimorphism 

 there is much difference of opinion, and we can only hint at 



