v] LATER PROGRESS 135 



to create a vacuum of their own, into which, as fast as 

 it is created, they jointly creep. This is in effect what 

 happens when two species become mutually depen- 

 dent. Here again the relation is at first a general 

 one, as between insects and flowers, but at the last 

 may get very special, as between the Yucca and its 

 moth. Both species here have lost independence. The 

 Yucca, for instance, has to be propagated artificially 

 in Europe, for when it was brought over the moth 

 was left behind, and so no seed can be set. At first 

 sight, then, such a system appears like a double 

 parasitism, and twice the evil that parasitism brings 

 should be its portion. This is not really so, for while 

 the true parasite takes what he can get and gives 

 nothing in return, here each pays the other willingly, 

 for services rendered. In extremes of parasitism 

 there is maximum waste; mutual aid (though it 

 implies mutual dependence) establishes minimum 

 waste. Moth and Yucca together constitute a system 

 which is harmonious and economical because division 

 of labour is at work : each does what it can do best 

 and gives of its superfluity to its partner. If the 

 two parts have sunk in the scale, yet by that very 

 sinking the beginnings of a new whole have sprung 

 up. They have lost in independence, but something 

 else the system formed by their combination- 

 has gained in harmony. Put in other words, their 

 own individuality has become impaired, but this is 



