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Show 1870.] IN THE GROWTH OF SALMON. 47 long period, seem to have lost sight of the value of several important points and consequent deductions. 1. That, in the natural condition, Salmon-fry do not all attain vigorous adolescence at the same period; still more so is their irregularity as regards perfect maturity. 2. That the periodical accession of the migratory impulse does not affect all alike, either as to time or season. 3. That those which do remain behind in the rivers for two seasons or more grow, relatively speaking, no larger than their juniors in age. 4. That the very fact of their abnormal retarded growth may account for several peculiarities as to organization, which divergence from the type has been put down to specific difference or hybridity. 5. That the admission of the uncertainty of the species (reared in the Gardens) strengthens the presumption of their being S. salar, when it has been demonstrated that their characteristic affinities are closer to that than to any other form. 6. That the belief in the hybridity of Salmonoids under natural conditions is, as yet, based on data less stable than the assertion that Salmon can abide long in fresh water: hence such an argument presents feeble opposition. 7. That not only does an occasional lengthened stay in fresh water accord with known habits, but, inversely, a more than ordinary protracted stay in sea-water occurs at times. In the instance mentioned (anted, p. 42), the majority of the return Grilse had grown no larger than those sojourning only a few months in the salt-water. This circumstance tells weightily in support of a temporary cessation of growth happening equally when Salmon dwell longer than the normal period either in the river or the sea. 8. That the fact of Salmon periodically tenanting sea and stream does not absolutely enforce migration under immediate penalty of death. The bodily improvement consequent on the change, however, is tantamount to its being needful to their ample development; ergo, causes checking the migratory impulse are coincident with the abeyance of growth. 9. That marine fish, other than migratory Salmonoids, have survived freshwater confinement. 10. That numerous instances can be adduced showing that fish of several sorts remain relatively feeble or thrive vigorously, according to limited space and nourishing food being withheld or granted to them. Salmon offer no exception. 11. That in well-attested cases fish confined to tanks are predisposed to variability-this notably in the Cyprinidse, which present no very distant family relationship to the Salmonidae, whence it may be assumed that such phenomena in the latter would be likely to supervene. 12. That in animals which exhibit peculiar phases of transformation or rapid changes at epochs of their life, exemplified in some Echinodermata and Insecta among invertebrates, and certain Batrachia* among vertebrates, physical agents play an important part in . * Vide Higginbottom's experiments (Phil. Trans. 1850, p. 434, pl. 32). |