[Illustration: FIG. 132.--Face view of recently hatched Flounder. (After S. R. Williams.)] "The degree of asymmetry can be correlated with the habit of the animal. Those fishes, such as the sole and shore-dwelling flounders, which keep to the bottom are the most twisted representatives of the family, while the more freely swimming forms, like the sand-dab, summer flounder, and halibut, are more nearly symmetrical. Asymmetry must be of more advantage to those fishes which grub in the mud for their food than to those which capture other fishes; of the latter those which move with the greatest freedom are the most symmetrical. "This deviation from the bilateral condition must have come about either as a 'sport' or by gradual modification of the adults. If by the latter method--the change proving to be advantageous--selection favored its appearing earlier and earlier in ontogeny, until it occurred in the stages of planktonic life. Metamorphosis at a stage earlier than this would be a distinct disadvantage, because of the lack of the customary planktonic food at the sea-bottom. At present some forms of selection are probably continually at work fixing the limit of the period of metamorphosis by the removal of those individuals which attempt the transformation at unsuitable epochs; for instance, at the time of hatching. That there are such individuals is shown by Fullarton, who figures a fish just hatched 'anticipating the twisting and subsequent unequal development exhibited by the head of Pleuronectids.' Those larvæ which remain pelagic until better able to compete at the sea-bottom become the adults which fix the time of metamorphosis on their progeny." (S. R. WILLIAMS.) So far as known to the writer, the metamorphosis of flounders always occurs while the individual is still translucent and swimming at the surface of the sea before sinking to the bottom. FOOTNOTES: [11] Cantor, Catal. Malayan Fishes, 1850, p. 87. Bowring, Siam, p. 155, gives a similar account of the battles of these fishes. [12] Francis Day, on Fear and Anger in Fishes, Proc. Zool. Society, London, Feb. 19, 1878, pp. 214-221. [13] Couch (Illustrations, etc., p. 305) says: "The faculty of giving forth brilliant light from the eyes is said to have been observed by fishermen in the blue shark, as in a cat." [14] Couch, "British Fishes," 1865, vol. iv. p. 172. [15] Himalayan Journals, vol. i. p. 80. [16] Hakluyt, vol. ii. p. 37. [17] Jerdon, "Madras Journal of Literature and Science," 1849, p. 143. [18] "Observations on the Past and Present Condition of Onjein," Journal of the Asiatic Society of Bengal, vi, p. 820. CHAPTER XII ADAPTATIONS OF FISHES [Illustration: FIG. 133.--Mad-tom, _Schilbeodes furiosus_ Jordan and Meek. Showing the poisoned pectoral spine. Family _Siluridæ_. Neuse River.]