Mr Shaw has frequently detected individual smolts, both of salmon and sea-trout, (though of the latter more particularly,) descending in some seasons as early as the end of March, and as late as the middle of June, and he has little doubt that some may make their way still earlier to the sea. These, of course, will be found in our tideways as small grilse, weighing one or two pounds, in April and May. The large parr, to which we have already alluded as occasionally met with in rivers, and which we regard as young salmon remaining (and in this forming exceptions to the normal rule) in fresh water throughout their third year, Mr Shaw, whose opinion we requested on the subject, coincides with us in thinking, "would, in all probability, be the first to quit the river after so long a residence there, when the season of migration approached. These, however, are not the only individuals of their kind which leave the river for the sea long before the month of May." A difference in the period of deposition will assuredly cause a difference in the period of hatching, and in this we agree with Mr Scrope; but we think that a late spawning, having the advantage of a higher temperature as the result of a more genial season, will be followed by a more rapid development, and so the difference will not be so great, nor expanded over so many months, as that gentlemen supposes. Finally, the vagrant summer smolts, to which we have before alluded, may consist of that small number of anomalous fry, which we know to assume the migratory dress and instinct soon after the completion of their first year. Although the excellence of a salmon's condition is derived from the sea, and all its increase of weight is gained there, yet few of these fish remain for any considerable length of time in marine waters. By a wonderful, and to us most beneficial instinct, they are propelled to revisit their ancestral streams, with an increase of size corresponding to the length of their sojourn in the sea. Such as observe their accustomed seasons, (and of these are the great mass of smolts,) return at certain anticipated times. Their periods are known, and their revolutions calculated. Such as migrate at irregular or unobserved intervals, return unexpectedly at different times. Their motions seem eccentric, because their periods have not been ascertained. But it is obvious that Mr Yarrell's diminutive examples already alluded to, could not have gone down to the sea with the great majority of their kind, during the spring preceding that in which they were captured; because, in that case, having remained a much longer time than usual in salt water, they would have returned as very large grilse instead of extremely small ones.