ON THE ORIGIN OF GENERA. 109 



seem to be intimately connected with difference of habit. The 

 increased convexity of carapace is an increased defense from fall- 

 ing objects — a danger to which land tortoises are far more subject 

 than the aquatic. Another protection, not needed by water tor- 

 toises so much as by terrestrial, is the faculty of closing one or 

 both free lobes of the plastron, as seen in the Cistudo, Sternothae- 

 rus, etc., or of portions of the carapace, as in Pixys, Cinixys, etc. 

 This might really have been produced by excessive tension on the 

 sternal and pelvic muscles while young, and while the sutures 

 were not fully interlocked. This, continued for a long time, 

 might have produced the result. Yet it is not easy to see what 

 protection the aquatic Kinosterna need in this respect, above the 

 Emydes of the same countries. The backs of these genera are 

 also as convex as are many of the terrestrial genera or Testu- 

 dinidae. 



I can not better express my views than by quoting the follow- 

 ing from the pen of the late Dr. Falconer. It is extracted from 

 one of his essays on the Elephantidre : * 



*' Each instance, however different from another, can be shown 

 to be a term of some series of continued fractions. When this is 

 coupled with the geometrical law governing the evolution of form, 

 so manifest in shells of the Mollusca, it is difficult to believe that 

 there is not in nature a deeper-seated and innate principle, to the 

 operation of which natural selection is merely an adjunct. 



"The whole range of the Mammalia, fossil and recent, can 

 not furnish a species which has had a wider geographical distri- 

 bution, and at the same time passed through a longer term of 

 time, and through more extreme changes of climatal conditions 

 than the mammoth. 



"If species are so unstable and so susceptible of mutation 

 through such influences, why does that extinct form stand out so 

 signally a monument of stability ? By his admirable researches 

 and earnest writings, Darwin has, beyond all his contemporaries, 

 given an impulse to the philosophical investigation of the most 

 backward and obscure braneli of the biological sciences of the 

 day ; he has laid the foundation of a great edifice ; but he need 

 not be surprised if, in the progress of erection, the superstructure 

 is altered by his successors, like the Duomo of Milan, from the 

 Roman to a different style of architecture. 



* See writings of Hugh Falconer, vol. ii (cd. by Murcbison). 



