158 



THE ORIGIN AND EVOLUTION OF LIFE 



of adaptation of form, which fall under the following great 

 principles of convergence and divergence: 



1. Divergent adaptation, by which the members of a primitive 



stock tend to develop differences of form while radiating 

 into a number of habitat zones. 



2. Convergent adaptation, parallel or homoplastic, whereby an- 



imals from different habitat zones enter a similar habitat 

 zone and acquire many superficial similarities of form. 



3. Direct adaptation, for example, in primary migration through 



an ascending series of habitat zones, aquatic to terres- 

 trial, arboreal, aerial. 



4. Reversed adaptation, where secondary migration takes a re- 



verse or descending direction from aerial to arboreal, 

 from arboreal to terrestrial, from terrestrial to aquatic 

 habitat zones. 



5. Alternate adaptation, where the animal departs from an orig- 



inal habitat and primary phase of adaptation into a sec- 

 ondary phase, and then returns from the secondary phase 

 of adaptation into a more or less perfect repetition of the 

 primary phase by returning to the primary habitat zone. 



6. Change of adaptation {function), by which an organ serving a 



certain function in one zone is not lost but takes up an 

 entirely new function in a new zone. 



7. Symbiotic adaptation, where vertebrate forms exhibit recip- 



rocal or interlocking adaptations with the form evolution 

 of other vertebrates or invertebrates. 



Law 



of 



Adaptive 



Radiation 



in the 



External 



Body 



Form 



It is very important to keep in mind that the body and 

 limb form developed in each adaptive phase is the starting 

 point of the next succeeding phase. 



Prolonged residence by an animal type in a single habitat 

 zone results in profound alterations in its chromatin and in 

 consequence the history of past phases is more or less clearly 

 recorded. 



Among the disadvantages of prolonged existence in one life 

 zone are the following: Through the law of compensation, dis- 

 covered by Geoffroy St. Hilaire early in the last century, every 

 vertebrate, in developing and specializing certain organs sacri- 



