10 THE DEPTHS OF THE SEA. (CHAP. 1. 
The individuals comprising a species have 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 circumstances, and is 
dangerous to the life of the individual. 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 which the specific 
characters are most strongly marked, is consequently 
slightly deflected, some characters being more strongly 
expressed at the expense of others. ‘This deflection, 
carried on for ages in the same direction, must even- 
tually 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 
difficult to form any idea of ten, fifty, or a hundred 
millions of years; or of the relation which such 
periods bear to changes taking place in the organic 
world. 
