64 DARWINISM AND HUMAN LIFE 



humming-bird beautifully poised before the flower 

 with almost invisibly rapid vibrations 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 neither case can we 

 doubt that the spreading influences are real, 

 though they pass beyond our ken. 



INTERRELATIONS BETWEEN FRESH-WATER MUS- 

 SELS AND FISHES. As a striking illustration of 

 the inter-linking of different forms of life, 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 period of temporary parasitism, at 

 the end of which there is a metamorphosis, they 

 drop off from the fish into the mud, often far 



