220 EVOLUTION, GENETICS, AND EUGENICS 



very rare, or wholly disappear." We know that the red clover 

 imported to New Zealand did not bear fertile seeds until humble-bees 

 were also imported. "The number of humble-bees in any district 

 depends in a great measure on the number of field-mice, which destroy 

 their combs and nests; and Colonel Newman, who has long attended 

 to the habits of humble-bees, believes that more than two-thirds of 

 them are thus destroyed all over England." Now the number of 

 mice is largely dependent, as everyone knows, on the number of cats; 

 and Colonel Newman says: "Near villages and small towns I have 

 found the nests of humble-bees more numerous than elsewhere, which I 

 attribute to the number of cats that destroy the mice." Thus we may 

 say, with Darwin, that next year's crop of purple clover is influenced 

 by the number of humble-bees in the district, which varies with the 

 number of field-mice; that is to say, with the abundance of cats! 



Scattering of seeds. — It is a fascinating chapter of natural history 

 which tells us how cross-polUnation is effected — here by a bee and 

 there by a butterfly, occasionally by a long-billed humming-bird 

 beautifully poised before the flower with almost invisibly rapid vibra- 

 tions of its wings, and occasionally by a slowly moving snail of epicure 

 appetite. But not less important is the part played by animals in the 

 scattering of seeds, and here again Darwin gives us the classic case of 

 fourscore seeds germinating out of a ball of mud from a bird's foot. 

 From one instance you may learn all, and see that much of Darwin's 

 work has been an eloquent commentary on that memorable saying 

 about the sparrow that falls to the ground. Such a simple event 

 literally sends a throb through surrounding nature; we can follow its 

 effects a few steps, just as we follow for a few yards the ripples made 

 when we throw a stone into a still lake; in either case can we doubt 

 that the spreading influences are real, though they pass beyond 

 our ken? 



Interrelations between fresh-water mussels and fishes. — As a 

 striking illustration of the inter-linking of different forms of Ufe, we 

 may take the case of the fresh-water mussels and their larvae. The 

 fertilised eggs develop in the outer gill-plate of the mother-mussel, and 

 minute bivalve larvae, called Glochidia, are formed. The mussel keeps 

 these within the cradle until a fresh-water fish — such as the minnow — 

 comes into the vicinity, and then she sets them free. In a way that 

 we do not understand, the simple constitution of the larvae is tuned 

 to respond to the presence of minnows and the like, and with snapping 

 valves they manage to fix themselves to their host. After a short 



