150 THE DOCTRINE OF DESCENT. 



suffice here also. The obscurity overshadowing- the 

 extraction of the birds is just beginning to clear up. 

 Why should not the origin of pterodactyls become 

 more distinct in the next few years ? 



A special difficulty 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 must not even say of the higher Ver- 

 tebrata alone, that its marvellous structure is well fitted 

 to excite the liveliest doubts as to descent and selection. 

 As a matter of fact, howe\'er, 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 lancelet 

 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 gra- 

 dation, and thus possess a representation of the manner 

 in which in the palseontological series the perfect organ 

 was gradually evolved from the simplest rudiments. 

 The lowest crabs present the simplest mechanism 

 imaginable, sensitive to light ; other crabs of higher de- 

 v^elopment 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 struc- 

 tures are represented, which clearly show that these 

 organs are also subject to the law of slow accumulation 

 and establishment of small advantages. 



With regard to the auditory and olfactory apparatus, 



