190 THE DOCTRINE OF DESCENT, 
Artificial selection—and here we may speak of design 
—produces perfection when, by mechanical and physio- 
logical labour (the latter especially by means of suit- 
able nutriment), it exercises the particular parts which 
are to be perfected, and propagates the advantages 
obtained. What we term natural selection is the epitome 
of the improvements acquired by specialization in the 
process of adaptation. The most faithful image of this 
gradually acquired specialization is afforded by the 
development of the individual, where from the undif- 
ferentiated, by constantly increasing differentiation, the 
mature animal is evolved in the plenitude of its physio- 
logical functions. That in the various animal groups 
certain grades of perfection are attained, is an uncon- 
troverted 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 indispensable, 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” (“ 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 
