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 irre- 
gularity 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 men- 
tioned (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, how- 
ever, 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 sur- 
vived 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. 
1]. That in well-attested cases fish confined to tanks are predis- 
posed to variability—this notably in the Cyprinide, which present 
no very distant family relationship to the Salmonidee, 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 transforma- 
tion or rapid changes at epochs of their life, exemplified in some 
Echinodermata and Insecta among invertebrates, and certain Batra- 
chia* among vertebrates, physical agents play an important part in 
* Vide Higginbottom’s experiments (Phil. Trans. 1850, p. 434, pl. 32). 
