Note 56 — Chapter V · EVOLUTION.

I had kept these tritons four years in the hope that they would breed; but in spite of their being subjected to great varieties of treatment—for months well supplied with food, and for months reduced almost to starvation—they never showed the slightest tendency to breed; another among the many illustrations of the readiness with which the generative system is affected even in very hardy and not very impressionable animals. Claparède observed the still more surprising fact that the Neritina fluviatilis (a river snail) not only will not lay eggs, but will not even feed in captivity. He attributes it to the stillness of the water in the aquarium, so unlike that of the running streams in which the mollusc lives. See Müller’s Archiv, 1857.