GENESTS. 231 



such as mammals and birds, the inevitable balancing of 

 assimilation by expenditure, establishes, for each species, an 

 almost uniform adult size ; and among creatures of these 

 kinds, (birds especially, in which this restrictive effect of 

 expenditure is most conspicuous), the connexion between 

 cessation of growth and commencement of reproduction, is 

 distinct. But we also saw ( 46) that where, as in the Cro- 

 codile and the Pike, the conditions and habits of life are such, 

 that expenditure does not overtake assimilation as the size 

 increases, there is 110 precise limit of growth ; and in creatures 

 thus circumstanced, we may naturally look for a compara- 

 tively indeterminate relation between declining growth and 

 commencing reproduction.* There is, indeed, among 



fishes, at least one case which appears very anomalous. The 

 male parr, or young of the male salmon, a fish of four or five 

 inches in length, is said to produce milt. Having, at this 

 early stage of its growth, not one hundredth of the weight 

 of a full-grown saliaon, how does its production of milt 

 consist with the alleged general law ? The answer must be 

 in a great measure hypothetical. If the salmon is (as it 

 appears in its young state) a species of fresh -water trout, 

 that has contracted the habit of annually migrating to the 

 sea, where it finds a food on which it thrives if the original 

 size of this species was not much greater than that of the 

 parr (which is nearly as large as some varieties of lake-trout 

 and river-trout) and if the limit of growth in the trout 

 tribe is very indefinite, as we know it to be ; then we 

 may reasonably infer, that the parr has nearly the adult 

 form and size of this species of trout, before it acquired 

 its migratory habit ; and that this production of milt, is, 



* I owe to Mr Lubbock an important confirmation of this view. After stat- 

 ing his belief, that between Crustaceans and Insects, there exists a physiological 

 relation analogous to that which exists between water-vertebrata and land-verte- 

 brata ; he pointed out to me, that while among Insects, there is a definite limit 

 f growth, and an accompanying definite commencement of reproduction, among 

 Crustaceans, where growth has no definite limit, there is no definite relation 

 between the commencement of reproduction and the decrease or arrest of grorf*h. 



