I suggested in former communications, that an essential point [p325] to ascertain, in any view of economy, or management, would be the proper balance of species; to discover what kinds would so live together that all the species might find food; might breed, each to its useful limits, so as to be serviceable to ourselves, the keepers or the flocks, and without hazard of the extermination of any kind. I may illustrate what is here meant, by a simple fact, in the ordinary economy of fresh water fishes in confinement. Pike and perch can live together, because the natural defences of the perch prevent the pike from exterminating the race, voracious as the enemy is. If trout and pike were confined in a narrow water, the trout would be destroyed. Or otherwise, it must be our object to ascertain, in an economical view, how to feed, by means of species that we do not desire to eat, those which we do cultivate for our own uses. This is a difficult question, which can only be overcome by time and experience; by knowledge; by knowledge, when we are in a state of entire ignorance; ignorance of every thing that relates to fishes, as great as if they were the inhabitants of another planet. This was one great source of difficulties with us in this case; and I, myself, must plead guilty, I fear, to a general recommendation of introducing every fish as a mere matter of trial; the result of which has been mischievous. The basse appears to have been the great enemy; to have eaten up the greater number of many species, and given no return. It has proved the pike of this pond. This could not have been foreseen; it is a caution for future speculators. Others will be discovered in the course of trial. It appears also that the common crab has proved destructive, probably by eating the spawn of larger fishes. From some enemy or other, the eels, which at first abounded to an incredible degree, have most materially diminished, and so have the shrimps. The latter, at least, appear to have been destroyed by the basse. Time and trial will teach us what to do in this case; in the infancy of ignorance, man might have supposed that he could keep wolves and sheep in one field, and have constructed a pen for foxes and fowls, rabbits and weasels. We must not accuse nature of our own ignorance.