NATUEAL SCIENCES OF PHILADELPHIA. 291 



reason to believe in such a relation than we have to believe that the same 

 bones in the hand of a man, wing of a bat, and fin of a porpoise, are related 

 to similar conditions of life. No one will suppose that the stripes on the 

 whelp of the lion, or the spots on the young blackbird, are of any use to these 

 animals, or related to the conditions to which they are exposed." 



The law of natural selection, however, has no doubt been a very important 

 agency in the production of organic types in different periods of the world's 

 history, but the part it has played in the determination of generic features 

 would appear to have been very small. 



In its first effect, — that of producing a structure adapted for a particular 

 purpose, — it would seem to have acted differently to produce the same results, 

 ahd hence not to have produced any of the more extended groups, as families, 

 where hundreds of species are identical in a single feature. Witness the 

 differences in diverse types of the tree-frogs, each type adapting its possessor 

 to an arboreal life : 



I. Claw-like, with globular base Hylid^. 



Leptopelis. 



II. Simple, obtuse-depressed at tip Ranid^. I aa and III a. 



III. With a terminal transverse limb Ranid^, //y^araraa et aff. 



Callula. 

 Brachymerus. 

 Hy lodes. 



iV. Bifurcate Batrachyla. 



Dendrobates. 

 Polypedates. 

 Rhacophorus. 

 The short foot of the Testudinidre, where one row of phalanges is omitted, 

 has been already alluded to. The gradual reduction of this set of bones, ac- 

 companying general modification of form in the increased convexitj of dorsal 

 region, as we leave the more aquatic and progress towards the terrestrial 

 tortoises, would seem to be intimately connected with difference of habit. 

 The increased convexity of carapace is an increased defence from falling ob- 

 jects, — a danger to which land tortoises are far more subject than the aquatic. 

 Another protection not needed by water tortoises so much as by terrestrial, 

 is the faculty of closing one or both free lobes of the plastron, as seen in 

 the Cistudo, Sternothaerus, 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 Uronyx and 

 Platythyrje 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 Testudinidaj. 



I cannot better express my views than by quoting the following from the 

 pen of the late Dr. Falconer. It is extracted from one of hia essays on the 

 Elephantida;.* 



" 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 geo- 

 metrical law governing the evolution of form, so manifest in shells of the 

 MoUusca, 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, cannot furnish a 

 species, which has had a wider geographical distribution, and, at the same 

 time, passed through a longer term of time and through more extreme changes 

 of climatal conditions than the mammoth. 



*See writings of Hugh Falconer, vol. ii. (Ed. by Murchison.) 



1868.] 



