THERE is a well known and marked divergence of view as to whether the various forms of our Salmonidae are to be regarded as merely local races and varieties, or as entitled to specific rank. This difference of opinion appears to be attributable to two main causes. One of these is the extraordinary plasticity of the whole family, as exemplified so strikingly in our ordinary brown trout, which are found in every variety of colour and marking; dark trout and light trout; greenish trout and brown trout; some with large red spots and some with many small spots; others again with none, and so on through an infinite variety of form. The other main source of difference arises from the loose and inaccurate use of the term ‘species.’ These questions have been partially examined in my previous chapter on the so-called ‘Ferox.’ It is sufficient here to say that in the further consideration of the relationship and differentiation of our various forms or races of the Salmonidae, it is proposed to accept Webster's definition of `species' as 'an ideal group of individuals resembling each other in essential characteristics and capable of indefinitely continued fertile reproduction through the sexes.'

As examples of extreme divergence of opinion on this subject we may note that whereas Gunther, 1866, gives no less than twelve separate species of the genus Salmo, Agassiz, 1834, already restricts the number to three ; while Dr. Day, held by many to be still our best authority,' is content with only two, that is, if the chars are left out of consideration. It is proposed, for present purposes, to treat only of the more important of the British salmones, i.e. the salmon, sea trout and common or brook trout, omitting such as the grayling, char, vendace, and others of the family. Those admitted by Agassiz were Salmo salar, S. trutta and S. fario; by Day, the salmon and the sea trout, land its several varieties, as the brook trout, Loch Leven trout, etc.'

We have seen above that in the common or brook trout and loch trout we find an infinite variety of colour and marking. The same is found to be the case, if in lesser degree, in the migratory forms usually included in the general term, sea trout. Some are thickly spotted, some with a few large spots, others with almost none. All, however, turn darker and more spotted with residence in fresh water, and assume various coloured markings at the time of reproduction. Experienced anglers will recall instances when they have found it difficult or impossible to decide with certainty to what category some particular capture should be assigned. We find that a sojourn in salt water has the effect of producing in the anadromous races a bright silvery sheen or hue which becomes quickly dulled and impaired on entering our rivers and lakes. On the other hand, the brown or brook trout, which frequent the lower or tidal reaches of rivers, gradually assume this silvery coat in greater or less degree according to the length of their stay and the brackishness of the water. In this state they are constantly caught by fishermen in salt water, sometimes at quite a considerable distance from the nearest fresh water.

Again it is to be remarked that some lochs, as for instance Loch Leven, Loch Craspuill, and others, produce trout of remarkable silvery appearance.

If coloration may thus be dismissed as valueless in the differentiation of forms, the same is found to be the case in the examination of structural characteristics. A full statement of the results arrived at, after careful examination of all the varied points of difference that have been held to constitute valid evidence of specific distinction, will be found in Dr. Day's British and Irish Salmonidae, where it is shown conclusively that in each and every case such characteristics have been found to be inconstant when the examination extended over a sufficiently large number of examples. All these forms or races agree in `essential characteristics.'

It has also been proved by careful and repeated experiment, notably by Sir James Maitland and Dr. Day at Howietoun, that the various forms, both anadromous and fresh water, are capable of breeding inter se, and that their progeny are fertile. It has further been established there that the descendants of an anadromous race, artificially reared and confined to fresh water, have themselves reproduced their kind. In November, 1885, Dr. Day exhibited at a meeting of the Linnean Society a par 51 inches in length, one of a shoal hatched at Howietoun in March, 1885, from parents themselves reared from ova and milt taken by Sir James Maitland from Teith salmon, which parents had, of course, never descended to the sea.

Apart from such experiments we have in the so-called land-locked salmon of Scandinavia and America races of fish, indistinguishable structurally from our British salmon, which spend their whole lives in fresh water. The term `land-locked' is inaccurate; for there is nothing to prevent these fish from descending to the sea, although there are certain obstructions in the way of their return. They appear to find the vast areas of fresh water to which they have access sufficient for all their requirements.

An important point to be remembered in the consideration of the question of relationship is the fact that in the youthful `par' stage of every race we find an almost absolute uniformity, so that it is with difficulty, if at all, that they can be separated with certainty. The cross bands or 'par-marks' are common to all, being lost in most cases on reaching maturity, although persisting throughout life in the case of the brook trout of some smaller streams; an instance, apparently, of arrested development through less favourable environment.

All the evidence, so far considered, points to the fact that the different races of our Salmonidae have certainly descended from one common ancestor, a conclusion leading to the difficult and much disputed question whether this common progenitor was of marine or fresh water origin. On this point opinion has always varied. Dr. Day, while not committing himself to a decided pronouncement, appears to incline to the view of a marine ancestry, and such is also the opinion of that eminent authority, Mr. Calderwood. One hesitates to oppose views of such weight; yet we are met with the fact that all of our salmones are only capable of reproduction in fresh water. It is well established that even in slightly brackish water their ova lose vitality and perish at once.

In the case of our migratory birds we find the homing instinct urging them, as the season of reproduction approaches, to seek the furthest limits of their migration, ever pressing in face of all difficulties towards those northern regions that are held to have been the place of origin of their ancestors. It would seem, then, somewhat strange to find, in the case of the Salmonidae, this homing instinct not only lost but absolutely reversed. A point rather in favour of the other contention should, however, be mentioned. Certain small streams and chains of lochans in the north of Scotland were, a number of years ago, full of small trout in their lower reaches only, below certain unsurmountable falls above which no trout was to be found until a number were carried up and placed above the obstructions. Since then they have multiplied exceedingly, and now afford excellent sport.

The life-history of that strange fish the eel, is in some ways exactly the opposite of that of the migratory Salmonidae. Modern research has proved that this fish, which passes its adult life in fresh water, returns once for all at the time of reproduction to great oceanic depths; the young eels ascending our rivers when they have assumed their final form, after passing through strange transitional changes in the sea.

One can hardly discuss this subject without adverting to the notable and interesting results of experiments in Australasia. There fish hatched from indubitable ova of English brown trout have, within the life-time of men yet middle-aged, developed to the size of 30 lbs. and upwards, have lost their red spots, assumed the silvery livery with black cross spots, and acquired the anadromous habit, descending to the sea and re-ascending to fresh water for purposes of reproduction; - changes of life and form, in a brief season, analogous to those accomplished in our hemisphere as the result of untold centuries.

We find, then, that all of these races satisfy entirely the requirements of the definition with which we started, and that they constitute a complete unbroken chain ; no man can say where one form ends and another begins. All merge imperceptibly into each other. In some, no doubt, we find the varying characteristics more fixed than in others; a burn trout is at once distinguishable, when in his native stream, from a sea trout. But the connecting links, as we have seen, are always to be found.

It may be asked what practical value there may be in such an enquiry and its conclusions. Apart from the reply that exact and definite knowledge is the very essence of true science, it must also be remembered that our laws have taken these fish under their special protection. Mr. Charles Stewart, in his Law of Scotland relating to Rights of Fishing, - a work of standard authority, - states: `A right of salmon fishing is a separate heritable estate, and being so, it is like all the lands and real rights in Scotland, vested in the Crown according to the feudal principle'....`the right of trout fishing is not a separate feudal estate, like salmon fishing.' And let it be remembered that the word `salmon' is held in law to include all varieties of migratory trout.

It will be seen that here we have material to raise many a knotty question, as, indeed, has already frequently occurred. Prosecutions for the taking of immature Salmonidae will be in the memory of many; and certain learned judges must at times have been astonished and perplexed by the conflicting nature of the evidence tendered by either side on the question of distinctive marks of specific differentiation. Well might the evidence be conflicting and perplexing if there be, in fact, no truly specific difference at all ! Such matters must be left to the lawyers; but looking to the importance of the subject and to the march of exact science, nowhere more marked than in our increasing knowledge of the life-history of the Salmonidae, the view may be permitted that, sooner or later, a comprehensive revision of these laws must be undertaken.