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60 to be proved as long as no explanation is forthcoming of the biological value of the colouration in the spring form V. levana. After the above lecture had been delivered, I received a recent paper by Dr. Brandes ('Der Saison-Dimorphismus bei einheimischen u. exotischen Schmetterlingen'; Halle, 1894), in which it is shown that a series of tropical butterflies are probably seasonally dimorphic in a manner similar to that which I have supposed to occur in Vanessa prorsa, the difference between the two forms being due to the adaptation of one generation to certain seasonal surroundings. Although these cases are not conclusively proved, I do not doubt that a seasonal dimorphism depending on adaptation exists. We must therefore distinguish between two kinds of seasonal dimorphism:—that which is called forth by the direct influence of temperature acting on the germ-plasm and on the growing scales, as well as an adaptive seasonal dimorphism, due to adaptation of both generations to different external surroundings. In the first instance, the temperature is the actual cause of the change in colouration; while in the second case it only serves as the stimulus which decides whether one or the other of two kinds of colouration shall appear.

NOTE VII (p. 28).

Rotifers, as is well known, produce two kinds of eggs, large ones that give rise to females, and small ones from which males arise. According to Maupas ('Sur le detérminisme de la sexualité chez l'Hydatina senta'; Compt. rend, T. 113, pp. 388–390), the same female always brings forth the same kind of eggs—either male alone or female alone; and it can be proved that whether a mother will produce daughter-forms which will bring forth male eggs, or daughter-forms which will produce female eggs, depends on the higher or lower temperature of the water in which the mother lives. We can make the mother produce alternately male-producing daughters or female-producing daughters; so