assume in virtue of the pressure of fluid from within. The form, moreover, varies according to the angle which the branch makes with the trunk and the relative sizes of the two vessels. There is consequently the smallest possible amount of friction of the blood on the walls of the vessels through which it circulates; and as the number of branches of the vessels is immense, we must acknowledge the importance of an arrangement which largely helps to render possible 'the working of the circulation with a minimum expenditure of vital energy and of the vascular tissue.'
NOTE V (p. 22).
See F. Merrifield, 'The Colouring of Chrysophanus phlaeas as affected by temperature,' The Entomologist, December, 1892, and December, 1893. The results arrived at by this accurate observer agree very well with those I obtained with regard to the same butterfly, a brief account of which is given in The Germ-Plasm: A Theory of Heredity. London, 1893, p. 399.
The fact that a direct action of the temperature is concerned in the case of Polyommatus (Chrysophanus) phlaeas and that the change of colour is not due to some special adaptation is shown by the observations of Fritze, who found the individuals of this species in South Japan were almost entirely black in the warmest part of the summer. Cf Fritze, 'Ueber Saison-Dimorphismus und Polymorphismus bei japanischen Schmetterlingen.' Berichte der naturf Ges. zu Freiburg i. Br., Bd. VIII, 1894, pp. 152–162.
While referring to Merrifield's valuable researches perhaps I may be allowed to call attention to the inconvenience resulting from recording temperature in degrees of Fahrenheit's scale in such works. Physicists and chemists have long since agreed to make use of the Centigrade scale in their writings; and it would be a great gain to scientific men in all the non-English countries if the zoologists of England and America would also adopt this usage. It is troublesome
