110 THE RATE OF GROWTH OF FOOD-FISHES. 



important fact that amongst Teleosteans, as a rule, the males 

 are considerably smaller than the females, and there is in some 

 cases (salmon, etc.) certain proof, and in others a great pro- 

 bability, that the males are mature at a much earlier date than 

 the females. Above the fishes also, many of the Amniota show 

 a tendency for an earlier maturation of the male products. 

 The bearing of this may be seen as follows : We may suppose 

 the primitive hermaphrodite chordate ancestor to have a 

 continual rhythmical and perhaps seasonal predominance of one 

 sex more or less directly dependent upon the environment 

 (cf. Yung's 1 observations upon Tadpoles). The tendency for a 

 seasonal repetition of the same environment would then have 

 the effect of causing a perfectly rhythmic sexual cycle, from 

 male to female, in each individual. This would be a case of 

 poly cyclic hermaphroditism. 



" This condition appears to be exemplified in the abnormally 

 hermaphrodite Teleosteans. The lengthening of the cycles 

 would result in a monocyclic hermaphrodite condition, as still 

 persistent in Myxine, and lastly, the reduction of the stage at 

 the commencement of the cycle to vanishing point in some 

 individuals (females), and hypertrophy of the former half in 

 others (males), would cause a hemi-cyclic dioecious species, as 

 exemplified by all other vertebrata. If we may regard this as, 

 in a general way, the line upon which the dioecious condition 

 has been evolved phylogenetically in the chordata, we have an 

 explanation of the facts of the earlier maturation of the males." 



If the maturation of the sexual organs takes place in a 

 certain district at a smaller size than is the case with the 

 average, then a " race " or variety of smaller fishes ensues. Thus 

 the plaice in the south of England, as far as statistics go, appear 

 to be smaller on an average at the attainment of maturity 

 than those, e.g., on the east coast of Scotland ; and Petersen 

 deduces facts to show that a smaller 'form' is present in the 

 Baltic than in other Danish waters ; while in Iceland and 

 certain other regions a ' giant race ' predominates. If we 

 assume that in all these ' forms ' the sexual organs mature at 



1 Arch. Zool. Expr., vii. Arch. Sci. Phys., Nat. xiv. 



