I50 THE DOCTRINE OF DESCENT. 



fice here also. The obscurity overshadowing- the extrac- 

 tion of the birds is just beginning to clear up. Why 

 should not the origin of pterodactyls become more dis- 

 tinct in the next few years? 



A special diiificulty is seemingly prepared for the 

 theory by the highly integrated organs, particularly by 

 the apparatus of the senses, with their very complex 

 mechanism. In truth, taking, for example, the eye of 

 the Vertebrata, we need not even say of the higher Ver- 

 tebrata alone; its marvellous structure is well fitted to 

 excite the liveliest doubts as to descent and selection. As 

 a matter of fact, indeed, the series of vertebrate animals 

 does not exhibit the series of lowly beginnings which we 

 must assume as having once existed. For the eye of the 

 fish is little inferior in complexity to the optic organ of the 

 mammal, whilst the lahcelet is completely eyeless, and 

 therefore affords no clue. 



In other orders of animals, however, we still see in 

 the systematic series of the present era every possible 

 gradation, and thus possess a representation of the man- 

 ner in which in the palaeontological series the perfect or- 

 gan was gradually evolved from the simplest rudiments. 

 The lowest crabs present the simplest mechanism im- 

 aginable, sensitive to light; other crabs of higher devel- 

 opment possess eyes somewhat more perfect, not only 

 sensitive to light, but capable of forming images, and 

 between these eyes and those of the decapodous crab, so 

 extremely perfect of their kind, a host of optic structures 

 are represented, which clearly show that these organs are 

 also subject to the law of slow accumulation and estab- 

 lishment of small advantages. 



With regard to the auditory and olfactory apparatus, 



