ORGANIC FORM AND ARCHITECTURE 675 



cases a developmental reason why two sets of peculiarities should 

 go together, being consequences of a common cause, or outcrops of 

 a common physiological tendency; but this has not been worked 

 out except in a few cases. In many cases it seems extremely improb- 

 able. Thus if we found a recent Marsupial skull, and could say— 

 from the geographical distribution perhaps, ©r from the dentition— 

 that it did not belong to the families of opossums or of Dasyures. 

 we could safely say that the female would have a pouch or marsu- 



FlG. 94. 

 To illustrate Homology. The wings of Pterodactyl (above), Bat (middle), 

 and Bird (below). From specimens. All are transformed fore-limbs. 

 R, radius; U, ulna; W, wrist; I-IV, fingers; P, patagium of skin; 

 PR, primary feathers; SEC, secondary feathers. 



pium. But a physiological or embryological correlation between the 

 skull and the pouch is highly improbable ! 



HOMOLOGY AND ANALOGY.— Organs that arise from the same 

 germinal layer have something in common; thus the scales of 

 Reptiles, the feathers of Birds, and the hairs of Mammals, so very 

 different from one another in structure, are alike in being entirely 

 epidermic differentiations, though nourished by the underljdng 

 dermis. But when organs develop in the same way from the same 

 germinal layer or layers, they have still more in common ; thus the 

 swim-bladder of a fish and the lung of a frog are both hollow out- 



