Page:The Effect of External Influences upon Development.djvu/63

Rh to have to convert degrees of Fahrenheit into Centigrade, and as a rule a remainder is left, so that one has constantly to work with fractions. Moreover no distinct picture is suggested to the mind of those unaccustomed to the use of Fahrenheit's scale.

NOTE VI (p. 24).

An observation of Dr. Adalbert Seitz, the well-known authority on butterflies, also seems to me to indicate that seasonal dimorphism, dependent on adaptation, may in fact occur. Dr. Seitz relates in his interesting 'Reiseskizzen' (Stettiner entomologische Zeitung, 1893, p. 27) that on the hills near Yokohama, in Japan, numerous butterflies are to be found flying about as late as November. Their 'lower surface is leaf-like, and this is the more noteworthy as the summer-generations of the same butterflies are different in this respect; the autumn-form of Grapta C. aureum has consequently been described as G. pryeri and Terias laeta and T. Jaegeri, which are only different generations of a seasonally dimorphic butterfly, have long been regarded as distinct species.' The author explains the adaptation by referring to the fact that at that time of year even the last remnants of the green leaves have disappeared, while the ground is strewn with withered leaves. It would be interesting to follow out this incidental allusion more carefully. It would not only show how marked and constant the differences between the two generations are, but would also prove the biological value of these differences; for whenever dimorphism depends on processes of selection, each of the forms is an adaptation, and must have a biological significance. For instance, if the colouration of the underside of the summer-generation were of no importance—that is, if the pattern might equally well be leaf-like—it would be difficult to see why the adaptation of the autumn-generation to dried leaves should not have extended to the other generation as well. This consideration is also valid in the case of Vanessa levana-prorsa. We cannot hold the adaptive nature of this seasonal dimorphism