THE CONQUEST OF THE DRY LAND 199 



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 breathing, 

 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, 

 leading to the linkage between flowers and 

 their visitors ; the third, the amphibian in- 

 vasion, leading to the evolution of wits and of 

 love. 



