398 THE CAUSES OF THE exe. 
up here on this black-board that you may remem- 
ber it—it is called Atavism ; it expresses this 
tendency to revert to the ancestral type, and comes — 
from the Latin word atavus, ancestor, 
Well, this Atavism which I shall speak of, is, as 
I said before, one of the most marked and striking 
tendencies of organic beings; but, side by side ~ 
with this hereditary tendency there is an equally 
distinct and remarkable tendency to variation. 
The tendency to reproduce the original stock has, 
as it were, its limits, and side by side with it there 
is a tendency to vary in certain directions, as if 
there were two opposing powers working upon the © 
organic being, one tending to take it in a straight 
line, and the other tending to make it diverge 
from that straight line, Ginst to one side and then 
to the other. 
So that you see these two tendencies need not 
precisely contradict one another, as the ultimate 
result may not always be very remote from what 
would have been the case if the line had been quite 
straignt. 
This tendency to variation is less marked in that 
mode of propagation which takes place non-sexu- 
ally ; itisin that mode that the minor characters of 
animal and vegetable structures are most com- 
pletely preserved. Still, it will happen sometimes, 
that the gardener, when he has planted a cutting 
of some favourite plant, will find, contrary to his 
expectation, that the slip grows up alittle different 
