Answer:
Darwin's theory of natural selection lacked the physical proof of the intermittent species as suggested by him. The fossils of such species had not been discovered by the time he wrote his book 'Origin of species'. In order to defend his theory, he pointed out the non-reliability on fossils.
Explanation:
The field of geology was just evolving at the time Darwin wrote his book, and no fossils supporting his theory has been discovered yet. He defended his theory by saying that the fossils cannot be depended on for the proof of evolution as the the process of fossilization depends on many factors, and the available fossils are very rare. The irregularities in the geological records, also suggested that the fossil records are not perfect.
Defending his theory, he shows how, if available. the fossil records can support his theory. The fossils should show the features of both the extinct (parent species ) species and the present species, as the species present in today's world have evolved from the ones that existed long before them through the process of natural selection.
The discovery of the Archaeopteryx in 1861 supported his theory and the process of evolution as it showed the features of both the reptiles and birds.
