INTRODUCTION ii 



two groups of animals, both terrestrial, were to have a 

 supply of food opened up to them in the water, it 

 seems probable that the group which had proved 

 itself the stronger by land would be more likely to 

 appropriate it. The past success of the European in 

 competition with the Hottentot gives us some reason 

 to believe that Europeans will occupy any country at 

 present uninhabited before the Hottentots will. But 

 we soon get out of our depth in such discussions as 

 this. 



Degrees of Adaptation to Aquatic 

 Conditions. 



How did Insects ever come to seek the water, seeing 

 that their mode of respiration is primarily adapted to 

 another element .-* We can see almost all the steps 

 of the adaptation on the shores of our rivers, lakes 

 and seas. We can see Dipterous larvae which, like 

 the " leather jacket " (the larva of the Daddy-long- 

 legs), burrow in the ground for their vegetable food, 

 and devour the roots of grasses. Other larvae of the 

 same family (Tipulidae) prefer moist earth in the 

 neighbourhood of streams. Others again live im- 

 mersed in water, or mud saturated with water, though 

 they come to the surface at times and push their 

 tails, which carry the spiracles, into the air. Some few 

 have become so completely aquatic that they seldom, 

 if ever, come to the surface, and all their supply of 

 oxygen is obtained from the water. Again we find 

 on any sandy sea-shore little Beetles, which hide in 

 the decaying seaweed. Some of the species only 



