The theory of evolution holds that living beings differentiated from each other over a very long period of time with linked, gradual modifications. If this theory were true, then numerous "intermediary species" should have lived in history linking different living species. For instance, if birds had indeed evolved from reptiles, then billions of half-bird/half-reptiles should have lived in history. Darwin knew that the fossil record ought to be full of these "intermediate transitory forms". Yet he was also well aware that no transitional form fossils were available. That was why he asked these troubled questions in his book The Origin of Species: "
You obviously know little about evolution...First, it's been over a 100 years since Darwin, so obviously we know more now.
Regarding transitional fossils:
"Due to the rarity of preservation and the likelihood that speciation occurs in small populations during geologically short periods of time, transitions between species are uncommon in the fossil record. Transitions at higher taxonomic levels, however, are
abundant"
"A transitional fossil is one that looks like it's from an organism intermediate between two lineages, meaning it has some characteristics of lineage A, some characteristics of lineage B, and probably some characteristics part way between the two. Transitional fossils can occur between groups of any taxonomic level, such as between species, between orders, etc. Ideally, the transitional fossil should be found stratigraphically between the first occurrence of the ancestral lineage and the first occurrence of the descendent lineage, but evolution also predicts the occurrence of some fossils with transitional morphology that occur after both lineages. There's nothing in the theory of evolution which says an intermediate form (or any organism, for that matter) can have only one line of descendents, or that the intermediate form itself has to go extinct when a line of descendents evolves.
To say there are no transitional fossils is simply false. Paleontology has progressed a bit since Origin of Species was published, uncovering thousands of transitional fossils, by both the temporally restrictive and the less restrictive definitions. The fossil record is still spotty and always will be; erosion and the rarity of conditions favorable to fossilization make that inevitable. Also, transitions may occur in a small population, in a small area, and/or in a relatively short amount of time; when any of these conditions hold, the chances of finding the transitional fossils goes down. Still, there are still many instances where excellent sequences of transitional fossils exist. Some notable examples are the transitions from reptile to mammal, from land animal to early whale, and from early ape to human"