HEREDITARY TRANSMISSION AND VARIATION. 89 



to reproduce the form of the parents. The proverb has 

 it that the thistle does not bring forth grapes; so, 

 among ourselves, there is always a likeness, more or less- 

 marked and distinct, between children and their parents. 

 That is a matter of familiar and ordinary observation. 

 We notice the same thing occurring in the cases of the 

 domestic animals — dogs, for instance, and their off- 

 spring. In all these cases of propagation and perpetu- 

 ation, there seems to be a tendency in the offspring to 

 take the characters of the parental organisms. To that 

 tendency a special name is given — and as I may very 

 often use it, I will write it up here on this black-board 

 that you may remember 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 hei'editary 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 pre- 

 cisely 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. 



