WILD THYME. 183 



record of the acreage under knotweed, or the 

 average yearly yield of the goosefoot crop. 



Now if, in such a case, any particular 

 plants of the infested species happen to be 

 protected against the intruder by some small 

 peculiarity of structure, they will survive 

 when their fellows perish, and a new species 

 will tend to be set up, possessing the pecu- 

 liarity which saved the lives of its ancestors. 

 This, or something like this, .is what must 

 have happened to the wild thyme and its 

 close relative the marjoram. They found 

 themselves exposed to some special danger 

 which did not threaten the other and larger 

 labiates ; and those alone survived which 

 possessed in a nascent form this fringe of 

 hairs concealing the nuts within. Perhaps the 

 selective agency at work was some small bird 

 or insect which could not tackle the larger 

 and harder nutlets of the dead-nettle and the 

 stachys : perhaps it was some creeping worm 

 against which the stiff and prickly stem hairs 

 of the bigger species formed an efficient 



