Why is fossil record hard to interpret?

Ever since and before Darwin's time, evolutionists have repeatedly insisted that all species of life have descended from a common ancestor as a result of a purely natural process of adaption to changing circumstances. So in lieu of the existence of any present proof for evolution, Darwin said that the only evidence of it would come from examining the fossil record. Thus if evolution did occur, one would expect to find a gradual series of fossils embedded in the rocks, from simple "almost life" chemicals,  to one-celled creatures, then two-celled creatures, on and on with greater complexity until you have the sponges and algae, the chordates and the trilobites and all of the invertebrates, then the vertebrates appearing animals with backbones, these last including fish, amphibians, reptiles, and man. That is only the Animal Kingdom; there is still the Plant Kingdom to be considered, with grapes and giant sequoia trees, carrots and flowers, potatoes and lawn grass, all of which supposedly evolved from the same common ancestor, according to evolutionists. Darwin stated that many of the great scientists of his time held the fossil record up as evidence against evolution, not for it.