BACKGROUND OF DARWINISM—THE WEB OF LIFE 211 
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-pollination 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 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, areformed. ‘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 
