GENESIS. 231 



BUch 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 grov/th 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 no 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 salmon, 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 k!s belief, tliat 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 conimenceraent of reproduction, araon^ 

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

 ^tween the commencement of reproduction and the decrease or arrest of gro //th. 



