ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 407 



caused more woody tissue to be formed on that side. In 

 the winter of 1 90 2, the bough was cut, and its autobiography 

 was interpreted with the aid of competent testimony that 

 was still available. 



Our practical studies shall be of the modifications of form 

 and appearance that belongs to racial, and not to indi- 

 vidual history. 



I. The re-adaptation of insects to aquatic life. 



It seems now quite clear that insects were primitively 

 terrestrial. They are covered with a tough chitinized skin, 

 well adapted to resist evaporation. They are provided 

 with a respiratory apparatus of distinctively aerial type. 

 They breathe through open spiracles, that lead to inter- 

 communicating air tubes (tracheae) within the body. As 

 adults, they all breathe free air, and are adapted only by 

 secondary makeshifts to aquatic life. It is only the 

 larvae of scattering groups that have become properly 

 aquatic, and able to breathe the air that is dissolved in the 

 water — all the larvae of a few small groups, and scattering 

 members of most of the larger orders. Among these, 

 therefore, we should be able to see the result of the fittinsr 

 of diverse forms to the new conditions. 



When, with the luxuriant development of the insect 

 group, the press of life on land crowded some insects back 

 into the water, the problem of getting air was the chief one 

 to be encountered. Its full solution lay in the development 

 of suitable respiratory apparatus. An impervious chiti- 

 nized skin perforated by open air tubes stood in the way of 

 ready re-adaptation. Adult insects merely adopted various 

 devices for carrying or otherwise obtaining free air when 

 in the water, without altering their mode of respiring it: 

 many insect larvae, also, get their air supply only at the 

 surface (fig. 238). But the softer and more plastic larvae, 



