Now, Mr Shaw's discovery in relation to these matters is in a manner twofold; first--he ascertained by a lengthened series of rigorous and frequently-repeated experimental observations, that parr are the early state of salmon, being afterwards converted into smolts; secondly,--he proved that such conversion does not, under ordinary circumstances take place until the second spring ensuing that in which the hatching has occurred, by which time the young are _two years old_. The fact is, that during early spring there are three distinct broods of parr or young salmon in our rivers.
1st, We have those which, recently excluded from the ova, are still invisible to common eyes; or, at least, are inconspicuous or unobservable. Being weak, in consequence of their recent emergence from the egg, and of extremely small dimensions, they are unable to withstand the rapid flow of water, and so betake themselves to the gentler eddies, and frequently enter "into the small hollows produced in the shingle by the hoofs of horses which have passed the fords." In these and similar resting-places, our little natural philosophers, instinctively aware that the current of a stream is less below than above, and along the sides than in the centre, remain for several months during spring, and the earlier portion of the summer, till they gain such an increase of size and strength as enables them to spread themselves abroad over other portions of the river, especially those shallow places where the bottom is composed of fine gravel. But at this time their shy and shingle-seeking habits in a great measure screen them from the observance of the uninitiated.
2dly, We have likewise, during the spring season, parr which have just completed their first year. As these have gained little or no accession of size during the winter months, owing to the low temperature both of the air and water, and the consequent deficiency of insect food, their dimensions are scarcely greater than at the end of the preceding October: that is, they measure in length little more than three inches.--(N.B. The old belief was that they grew nine inches in about three weeks, and as suddenly sought the turmoil of the sea.) They increase, however in size as the summer advances, and are then the declared and admitted parr of anglers and other men.