238 THE PERPETUATION OF ADAPTATIONS 



more variations than do other types. Some modifications were 

 capable of great development; these were " lucky strikes" so to 

 speak, in evolution, which enabled the organisms to respond 

 more quickly or more adequately to changes in environment. 

 One of the greatest of these probably was the development of 

 metameric structure; another was the development of the in- 

 ternal osseous skeleton; still another was the development of 

 air tubes or tracheae for respiration, and others will occur to 

 every student. 



