THE CONQUEST OF THE DRY LAND 199 
vance guard of the third great invasion. They 
were the first animals to have fingers and toes 
(the paired fins of fishes are limbs, but they 
have no digits), the first animals to have a 
three-chambered heart (though the mud-fishes 
come near this), the first animals to have true 
lungs (though some fishes like the mud-fishes 
use their swim-bladder to help them in breath- 
ing, and it is no doubt the forerunner of a 
lung), the first animals to have a movable 
tongue, and the first backboned animals to 
break the silence of animate nature by having 
a voice. 
Besides the three invasions or colonisations 
which we have mentioned, there were no 
doubt others, like that which led to land-crabs 
and wood-lice (terrestrial crustaceans), or 
that which led to snails and slugs (terrestrial 
molluscs). 
But in thinking of the conquest of the land, 
we will not go far wrong if we give prominence 
to the idea of three great invasions—the first, 
the worm invasion, leading to the making of 
fertile soil; the second, the insect invasion, lead- 
ing to the linkage between flowers and their 
visitors; the third, the amphibian invasion, 
leading to the evolution of wits and of love. 
