In the birds as in the mammals, and quite separately in the two groups, we see the evolution not only of certain structural characters such as division of heart, compactness of skeleton, increase of brain-size, not only of physiological characters like warm-bloodedness or efficiency of circulation, but also of various psychical characters. The power of profiting by experience becomes greater, as does that of distinguishing between objects; and there is most markedly an increase in the intensity of emotion. It has somehow been of advantage, direct or indirect, to birds to acquire a greater capacity for affection, for jealousy, for joy, for fear, for curiosity. In birds the advance on the intellectual side has been less, on the emotional side greater: so that we can study in them a part of the single stream of life where emotion, untrammelled by much reason, has the upper hand. BIBLIOGRAPHY Chance, E., ’22. “The Cuckoo’s Secret.” London, 1922. Darwin, C., ’71. “The Descent of Man, and Selection in Relation to Sex.” London, 1871. Groos, K., ’98. “The Play of Animals.” New York, 1898. Howard, E., ’20. “Territory in Bird Life.” London, 1920. Hudson, W. H., ’12. “The Naturalist in La Plata” (5th Ed.). London, 1912. Huxley, J. S., ’14 and ’23. (Courtship in Birds) Proc. Zool. Soc., 1914, and Proc. Linn. Soc., 1923. Kirkman, F. B. (ed.), ’10. “British Bird Book.” London, 1910. Levick, G. M., ’14. “Antarctic Penguins.” London, 1914. Massingham, H. J., ’23. “The Ravens.” _Nation and Athenæum._ London, 21st April, 1923. Selous, E., ’01. “Bird Watching.” London, 1901. Selous, E., ’05. “Bird Life Glimpses.” London, 1905. FOOTNOTE: [26] Massingham, ’23. IV SEX BIOLOGY AND SEX PSYCHOLOGY SEX: THREE WAYS That body has for soul an air-balloon Which drifts with every spiritual blast, Doomed, swollen thing! to leak or burst at last Though overmuch aspiring toward the moon. This other soul, below the animal, Bloating and coating body’s baser parts With the manure of its desires and arts, Helps flesh to grow still more corporeal. I pray that I may still inhabit earth, Where grass invites the foot, and roses smell; Yet shall I lead my body on to dwell In the eternal land of second birth, If, nought contemned, each part of being’s whole Is taken up in my transmuting soul. SEX BIOLOGY AND SEX PSYCHOLOGY[27] “And now I see with eye serene The very pulse of the machine.” --W. WORDSWORTH.