Objections to Darwin's theory

• Darwin needed a good theory of heredity

One of the more sophisticated objections to Darwin's theory was that it lacked a satisfactory theory of heredity. There were various theories of inheritance at that time, and all of them are now known to be wrong.

Darwin preferred a 'blending' theory of inheritance, in which the offspring blend their parental attributes; for example, if a red father male mated with a white female, and inheritance “blended”, the offspring would be pink. One of the deepest hitting criticisms of the theory of natural selection pointed out that it could hardly operate at all if heredity blended.

• It is sometimes difficult to imagine gradual evolution

One of the main objections to natural selection was that there are gaps between forms in nature that could not be crossed if evolution was powered by natural selection alone. The anatomist St George Jackson Mivart (1827 - 1900) listed a number of organs that would not (he thought) be advantageous in their initial stages.

In Darwin's theory, organs evolve gradually, and each successive stage has to be advantageous in order that it can be favored by natural selection. Mivart retorted that although for a bird a fully formed wing, for example, is advantageous, the first evolutionary stage of a tiny proto-wing, might not be.

• It is sometimes difficult to imagine evolution if it occurs by chance

Another objection was the misunderstanding that Darwin's theory explained evolution by chance. Biologists who accepted Mivart's criticism sought to get round the difficulty by imagining processes other than selection that could work in the early stages of a new organ's evolution.

Most of these processes belong to the class of theories of 'directed mutation': they suggested that offspring, for some unspecified reason to do with the hereditary mechanism, consistently tend to differ from their parents in a certain direction. In the case of wings, the explanation by directed variation would say that the wingless ancestors of birds somehow tended to produce offspring with proto-wings, even though there was no advantage to it.

The image opposite is of Archaeopteryx, the first species to develop feathered wings: critics of Darwin doubted how wings like this could evolve gradually, being advantageous at every stage.