202 



THE ORIGIN AND EVOLUTION OF LIFE 



developed by Louis Dollo, of Brussels. This is applied hypo- 

 thetically to the evolution of the existing leatherbacks (Sphar- 



ABYSSAL 



Fig. 78. Chelonia. Diagram Illustrating the Alternate Habitat Migration 

 OF THE Ancestral "Leatherbacks," SpHARGiD.i. 



DoUo's theory is that these animals originate in armored land forms with a solid bony 

 shell, and pass from the terrestrio-aquatic into the littoral and then into the pelagic 

 zone, in which the solid bony shell, being no longer of use, is gradually atrophied. After 

 prolonged marine pelagic existence these animals return secondarily to the littoral 

 zone and acquire a new armature of rounded dermal ossicles which develop on the 

 upper and lower shields of the body. The animals (Sphargis) then for a second time 

 take up existence in the pelagic zone, during which the dermal ossicles again tend to 

 disappear. 



gidae), an extremely sj^ecialized type of sea turtles. It is be- 

 lieved that after a long period of primary terrestrial evolution 



^^^_^ ^__^ ii^ which the ancestors of 



these turtles acquired a firm, 

 bony carapace for land de- 

 fense, they then passed 

 through various transitions 

 into a primary marine phase 

 during which they gradually 

 lost all their first bony arma- 



FiG. 79. The Existing "Leatherback" ture. Following this sea 



The Existing "Leatherback" 

 Chelonian Sphargis. 



In this form the solid armature adapted to a 

 former terrestrial existence is being replaced 

 by a leathery shield in which are embedded 

 small polygonal ossicles. After Lydekker. 



phase the animals returned 

 to shore and entered a 

 secondary littoral, shore-liv- 

 ing phase, also of long dur- 

 ation, in course of which they developed a second bony 

 armature quite distinct in plan and pattern from the first. 



