58 PHYSOSTOMI. 



than one manner, as the nnptial season, the effects of temporary emotions, of age, 

 or the state of the creature's health, or its food. It has been observed that a 

 residence in sea or brackish water causes the fish to assume a general silvery or 

 steel colour, with or without black spots, and that these latter are not surrounded 

 by a light circle. Clear water in rapid rivers or lakes,* especially when the 

 bottom is pebbly, is similarly seen to contain somewhat silvery fishes, with black 

 X-shaped spots. Sir William Jardine remarked that a variety very frequent 

 among trout in small Alpine lochs in Scotland had large dark or red spots placed 

 in a pale or clear surrounding field, these marks being very large, while the 

 principal part of the spotting was confined to the centre of the body. The colour, 

 depth, and character of the water also have an influence on the fish, the presence 

 of moss and peat, or a muddy bottom, usually causing a dark tint, while some 

 captured in dark holes or caves have been seen nearly black. The colours of the 

 Salrnones may be shortly summed up as silvery, with or without black spotsf 

 among the marine, and some resident in large clear pieces of water, as lochs or 

 rivers : more or less speckled with black and red when non-migratory and living 

 in fresh waters : while should the race be small, a persistence of the transverse bars 

 or bands on the body, which are present in most of the young, may be observed 

 even in adults. 



Irrespective of changes in colour externally, a difference in food may occasion 

 it in the flesh of these fish, whether such alteration in diet is due to choice or to 

 necessity. Thus Crustacea and their allies may be absent from the locality they 

 frequent, or if present the fish may not relish that food so much as some other 

 which exists in the water. In certain rivers there are trout with white and others 

 with red flesh, the two forms being in good health and equally delicate for the 

 table. This has also been observed in the American charr, Salmo fontinalis, 

 introduced into this country, and in which it has been clearly traceable to the 

 food it lives upon. 



3. The form of the preopercle in adult fish. The shape of this bone varies in 

 species belonging to the true Salrnones with the age of the fish, while it has 

 likewise been remarked that in some races the development of its lower limb is 

 much more pronounced that it is in others obtained from different localities. 

 This, however, merely shows that in the marine as in the fresh water races there 

 are fish subject to variations in the shape of this bone. The limb is very short in 

 the young, elongating with age in some forms, but not so in others ; while an 

 arrest of development may easily take place even on opposite sides of the head 

 of a specimen, which were this the sole criterion of species, might, and 

 sometimes does, show Salmo trutta on one side and S. albus, or brachypoma, 

 on the other. Thompson was of opinion that differences in the form of this bone 

 may sometimes be due to sex. 



4. The width and strength of the maxillary in adult fish. This will depend upon 

 the food which the fish has subsisted upon during its lifetime. If this organ has 

 been much called into action, it will greatly exceed in size and strength what is 

 found in examples in which it has been less employed. It appears to be almost 

 invariable that the lower jaw is more developed in the male than in the female, 

 besides which in the former it is often provided with a hook at its extremity. 



5. The size of the maxillary and of the vomerine teeth. The same remarks are 

 suitable to this form of variation as apply to the maxillary bone. 



6. The arrangement and permanence of the vomerine teeth. These vary so 



* Percy St. John, in Wild Sports of the West, p. 240, remarks that he " never observed the 

 effect of bottom soil upon the quality of fish so strongly marked as in the trout taken in a small 

 lake in the county of Monaghan. The water is a long irregular sheet, of no great depth, one 

 shore bounded by a bog, the other by a dry and gravelly surface. On the bog side the trout are 

 of the dark and shapeless species peculiar to moory loughs, while the other affords the beautiful and 

 sprightly variety, generally inhabiting rapid and sandy streams. Narrow as the lake is, the fish 

 appear to confine themselves to their respective limits : the red trout being never found upon the 

 bog moiety of the lake, nor the black where the under surface is hard gravel." 



f It cannot be admitted that the black X-shaped spots are due to the influence of salt water, as 

 we see them present in our strictly fresh water grayling, Thymallus. 



