10 SEXUAL DIMORPHISM 



breeding takes place. The individuals which lived in 

 estuaries would breed together, and not with those that lived 

 always in the sea. Thus any modification produced in the 

 one group would remain confined to that group, and by 

 interbreeding within that group would be transmitted to all its 

 members. It is difficult in the present state of our knowledge 

 to say how modifications of the kinds here in question are 

 determined. There are indications that continuous modifica- 

 tion takes place in successive generations without any direct 

 stimulus that can be detected. We must remember that the 

 development of the individual is the growth and multiplica- 

 tion of groups of differentiated cells. This development is 

 controlled partly by heredity, partly by surrounding con- 

 ditions, but the development of every part and every organ 

 is to some extent independent. The facts indicate that a 

 particular part or organ may, for some unknown reason, 

 obtain increased nourishment and develop with increased 

 vigour, or, on the contrary, may show diminished vigour, and 

 that the change may be progressive in successive generations. 

 It would seem that ultimately the effect must be due to 

 external conditions acting upon the properties of living 

 matter. But the action is evidently very indirect, and the 

 processes involved are so complicated and so recondite that 

 at present we know nothing about them. We have indica- 

 tions of the influence of external conditions in the differences 

 in the same species in different geographical areas. Thus the 

 flounder in the Mediterranean has scarcely any tubercles, 

 while in the Baltic and Arctic regions they are excessively 

 developed, so that nearly the whole skin is covered with 

 them. Whether this is due to the cold or not we do not 

 know, but it is a fact that the plaice also shows greater 

 roughness of the scales in the north than in the south. The 

 difficulty is that this effect of northern latitudes is not 

 observed in fishes of all families, and we have to face the 



