igo THE DOCTRINE OF DESCENT. 



of the improvements acquired by specialization in the 

 process of adaptation. The most faithful image of this 

 gradually acquired specialization is afforded by the de- 

 velopment of the individual, where from the undifferen- 

 tiated, by constantly increasing differentiation, the mature 

 animal is evolved in the plenitude of its physiological 

 functions. That in the various animal groups certain 

 grades of perfection are attained, is an uncontroverted 

 fact; but every closer investigation shatters the idol of 

 design. The organism of the bird might induce us to 

 consider it, in the abstract, as modified for the purpose 

 of flight. But if design be allowed to watch over the 

 good flyers, the idea of design must be abandoned with 

 respect to the non-flyers, and, if some idea is indispensa- 

 ble, adaptation must have its due. Herewith the whole 

 theory is broken down, and it will be the same in every 

 other case. 



How organic perfection stands with reference to the 

 idea of design, has been acutely and clearly expressed by 

 the author of the " Unconscious " (" Das Unbewussten "). 

 The theory of descent teaches that there is no inde- 

 pendence of the conditions co-operating, in an organic 

 phenomenon; rather that its increasing divergence from 

 a common neutral point was an effect of the same causes. 

 The theory of selection makes us acquainted with one 

 of these causes, and unquestionably the most important 

 as one, which, by purely mechanical compensative phe- 

 nomena, produces advantageous results. The theory of 

 descent merely casts doubts on the teleological principle 

 by withdrawing the basis for positive proof, but the 

 doctrine of selection sets it directly aside, so far as it is 

 able to extend its explanation. For natural selection 



