150 TIIE 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, thatits marvellous structure is well fitted 
to excite the liveliest doubts as to descent and seleetion. 
As a matter of fact, however, 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 paleontological 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- 
velopment 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 ate 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, 
