1 THE DEPTHS OF THE SEA. [chap. i. 



The individuals comprising a species liave a definite 

 range of variation strictly limited by the circum- 

 stances under which the group of individuals is 

 placed. Except in man, and in domesticated animals 

 in which it is artificially increased, this individual 

 variation is usually so slight as to be unappreciable 

 except to a practised eye ; but any extreme variation 

 which passes the natural limit in any direction 

 clashes in some way with surrounding circum- 

 stances, and is dangerous to the life of the indivi- 

 dual. The normal or graphic line, or 'line of 

 safety,' of the species, lies midway between the 

 extremes of variation. 



If at any period in the history of a species the 

 conditions of life of a group of individuals of the 

 species be gradually altered, with the gradual change 

 of circumstances the limit of variation is contracted 

 in one direction and relaxed in another ; it becomes 

 more dangerous to diverge towards one side and 

 more desirable to diverge towards the other, and the 

 position of the lines limiting variation is altered. 

 The normal line, the line along Avhicli the specific 

 characters are most strongly marked, is consequently 

 slightly deflected, some characters being more strongly 

 expressed at the expense of others. This deflection, car- 

 ried on for ages in the same direction, must eventually 

 carry the divergence of the varying race far beyond 

 any limit within which we are in the habit of 

 admitting identity of species. 



But the process must be infinitely slow. It is diffi- 

 cult to form any idea of ten, fifty, or a hundred mil- 

 lions of years ; or of the relation which such periods 

 bear to changes taking place in the organic world. 



