398 THE CAUSES OF THE xi 



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, first 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 

 straight. 



This tendency to variation is less marked in that 

 mode of propagation which takes place non-sexu- 

 ally ; it is in 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 a little different 



