According to Darwin, biological evolution is open-ended and not targeted. That means evolution cannot be understood in terms of its goal, instead in terms of its starting point. A causal chain of argumentation must be built up from the starting point. (Example: feathers were not created to allow the birds to fly.)
Ernst Mayr wrote about this in his great work, The Growth of Biological Thought:
Darwin’s theory uncompromisingly rejects the existence of definitive factor in the cause of evolutionary change
(German: Ernst Mayr 2002: Die Entwicklung der biologischen Gedankenwelt, Berlin et al., p. 417)
According to Darwin, there are 2 steps of evolution, the 1st being genetic variability, and the 2nd the natural selection in every generation of living beings. A long-term goal for evolution cannot be in any way pursued, since intermediate generations would have died out long before the goal could be achieved.
The reason for this is that the half-finished organs, (e.g. half-finished feathers for flying) would not lead to selection success for the individuals that possess them.