The Story of Evolution 85 



which led eventually to the important linkage between flowers 

 and their insect visitors, and the invasion due to air-breathing 

 Amphibians, which led eventually to the higher terrestrial animals 

 and to the development of intelligence and family affection. Be- 

 sides these three great invasions, there were minor ones such as 

 that leading to land-snails, for there has been a widespread and 

 persistent tendency among aquatic animals to try to possess the 

 dry land. 



Getting on to dry land had a manifold significance. 



It implied getting into a medium with a much larger supply 

 of oxygen than there is dissolved in the water. But the oxygen 

 of the air is more difficult to capture, especially when the skin 

 becomes hard or well protected, as it is almost bound to become 

 in animals living on dry ground. Thus this leads to the develop- 

 ment of internal surfaces, such as those of lungs, where the 

 oxygen taken into the body may be absorbed by the blood. In 

 most animals the blood goes to the surface of oxygen-capture; 

 but in insects and their relatives there is- a different idea 

 of taking the air to the blood or in greater part to the area of 

 oxygen-combustion, the living tissues. A system of branch- 

 ing air-tubes takes air into every hole and corner of the insect's 

 body, and this thorough aeration is doubtless in part the 

 secret of the insect's intense activity. The blood never becomes 

 impure. 



The conquest of the dry land also implied a predominance 

 of that kind of locomotion which may be compared to punting, 

 when the body is pushed along by pressing a lever against a hard 

 substratum. And it also followed that with few exceptions the 

 body of the terrestrial animal tended to be compact, readily lifted 

 off the ground by the limbs or adjusted in some other way so that 

 there may not be too large a surface trailing on the ground. An 

 animal like a jellyfish, easily supported in the water, would be 

 impossible on land. Such apparent exceptions as earthworms, 

 centipedes, and snakes are not difficult to explain, for the earth- 



