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. 



