It happened most curiously that a Mr. Jackson (I believe a government officer of the United States) was, about this same period, amusing himself by fishing at Salsalita, and caught two viviparous fish, a male and a female. On cutting open the female, to obtain a piece of the belly for bait, he, like myself, was astonished at seeing a whole bevy of tiny fish come scrambling out, and at first imagined, as I did, that they had been swallowed. He immediately wrote a letter to Professor Agassiz, sending the mutilated fish, having previously satisfied himself that they had not been devoured, and stating at length his singular discovery. The professor was astonished, and disbelieved the possibility of the fish being viviparous, imagining some error had crept into the statement sent him by Mr. Jackson; but other fish in a similar state were subsequently obtained by Mr. Carey, and forwarded to the learned professor. The fact was then most undeniably established, that this and many other species were strictly viviparous. I have spoken of this at some length, because it is a curious coincidence that the same fact should have been discovered by two men a long distance apart, about the same date, and by both in the same way,—by sheer accident. Now we come to a ticklish question: how are the young fish vitalised in the abdomen of the mother? In this case I shall adopt what I conceive to be the most straightforward course, which is candidly to give my own thoughts, and solicit from abler, older, and better physiologists their opinions or theories—for I sincerely think this is a question well worth careful investigation. I believe the ova, after impregnation, at first goes through the same transformations in the ovarium as it would do, supposing it to have been spawned and fecundated in the ordinary spawning-bed, but only up to a certain point; then, I think, the membrane enfolding the ova, that have by this time assumed a fishlike type, takes on the character and functions of a placental membrane, and the young fish are supplied by an umbilical cord, just as in the case of a fœtal mammal. But a third change takes place. There can be no doubt that the young fish I cut out, and that swam away, had breathed before they were freed from the mother; hence I am led to think that, a short time prior to the birth of the young, sea-water has access to this marsupial sac, washes over the infant fish, the gills assume their normal action, and the regular systemic circle is established. Maturity attained, the umbilical attachment snaps, and the little fish, perfect in every detail of its organisation, is launched into the deep, to brave its many perils, and shift for itself. The strong transverse muscles attached to the powerful sphincter (constituting the genital opening acting from the abdominal walls), I imagine, are in some way concerned in admitting the sea-water, and it appears to me a contrivance admirably adapted to effect such a purpose; but how