292 THE INDUCTIONS OP BIOLOGY. 



the fullest activity of the reproductive function arises as 

 growth ceases : speaking generally, at least ; for though this 

 relation is tolcrahly definite in the highest orders of animals 

 which multiply by gamogencsis, it is less definite in the lower 

 orders. This admission does not militate against the hypo 

 thesis, as it seems to do ; for the indefmiteness of the relation 

 occurs where the limit of growth is comparatively indefinite. 

 We saw ( 46 ) that among active, hot-blooded creatures, such 

 as mammals and birds, the inevitable balancing of assimila 

 tion 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 expendi 

 ture is most conspicuous), the connexion between cessation 

 of growth and commencement of reproduction is distinct. 

 But we also saw (4G) that where, as in the Crocodile and 

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

 expenditure does not overtake assimilation as size increases, 

 there is no precise limit of growth; and in creatures thus 

 circumstanced we may naturally look for a comparatively 

 indeterminate relation between declining growth and com 

 mencing 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 great measure hypothetical. If the salmon is (as it ap 

 pears to be in its young state) a species of fresh- water trout 



* I owe to Mr. (now Sir John) Lubbock an important confirmation of 

 this view. After statins: his belief that between Crustaceans and Insects 

 there exists a physiological relation analogous to that which exists between 

 water- vcrtebrata and land-vertebrata, he pointed out to me that while among 

 Insects there is a definite limit of growth, and an accompanying definite com 

 mencement 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 growth. 



