The traditional scheme for classifying organisms is that pioneered by the Swedish botanist Linnaeus (1707-1778). Like most naturalists of the 18th century, Linnaeus believed that God had created the world, and it was within that framework that he pursued his research. Initially he supposed that the bounds within which organisms varied had remained fixed over time – bounds which were defined by species. However, he changed his mind when he saw that new species could sometimes arise through the crossing of species, and presumably many existing species could have arisen through cross-fertilisation in the wild. He therefore proposed that the fundamental unit in nature was the genus rather than the species. Later he became open to the idea that genera too could have been the outcome of hybridisation. The originally created ‘kinds’ could have possessed a capacity for variation wide enough to encompass whole families.

In his Origin of Species Darwin argued against the fixity of species, and it was therefore with some surprise that he learned, eight years after publishing his book, that Linnaeus himself had not espoused the concept. In arguing against immutability Darwin had supposed that he had been arguing against the concept of creation itself, and much of the force of his argument had depended on this association (see Evolution? What evolution?). In a sense, the species was more of a fundamental unit for him than it was for Linnaeus, since he thought that if he could explain the origin of species (by showing that one species derives from another), this was tantamount to explaining the origin of all species. Linnaeus’s view was more nuanced. Depending on the organism under consideration, he considered that the fundamental unit might be the species alone, the species and genus together, or the species, genus and family together, with a hierarchy of created difference above and a hierarchy of subsequent diversification below.

Linnaeus, from his theistic perspective, believed there were limits to how far organisms could vary. Darwin, from his atheistic perspective, believed there were none. Both scientists cited the present-day evidence of hybridisation to support their views. However, Darwin needed the further support of the fossil record, and that he did not have. In chapter nine of the Origin of Species he admitted:

The number of intermediate varieties, which have formerly existed on the earth, [must] be truly enormous. Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory.

In mitigation, Darwin argued that the gaps between species and the suddenness with which species first appeared as fossils were illusory, due to the fact that the geological record was discontinuous and inadequately sampled. When it was better known, he predicted, many of the gaps would disappear.

Was Darwin right?

In the past hundred and fifty years our knowledge of the fossil record has grown enormously. We can now trace the history of many groups of animals, such as dinosaurs, turtles, horses, cats – to name just some of the more familiar examples. One of the animal groups for which we have an exceptionally detailed record is the trilobites, as summarised in the next diagram (click the image).

In this example, we see trilobites diversifying over time from a small number of orders (the Redlichiina and so on). The orders each have distinct characteristics, but they are also sufficiently alike to be recognisably all trilobites, so it is reasonable to suppose that the whole class is genealogically related. How they came to be so diverse, however, is far from clear. The pattern shows evolution but not Darwinian evolution. It is not clear how any of the Ordovician orders arose (presumably from precursors in the Cambrian), how any of the Cambrian orders arose (presumably from trilobites in the Precambrian), or how the trilobites as a distinct class arose (the absence of precursors suggests that evolution may not be the answer). There is no finely graduated organic chain. When trilobites first appear, they are already diverse, and the rapid transitions which give rise to new orders seem more like the outcome of genetic programming than unpremeditated mutations.

The same is true of other groups. Dinosaurs (which appear to have multiple origins) arise out of nowhere. So do turtles and horses. In the oft-quoted words of Stephen Jay Gould:

The extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology. The evolutionary trees that adorn our textbooks have data only at the tips and nodes of their branches; the rest is inference, however reasonable, not the evidence of fossils.

S. J. Gould, The Panda’s Thumb, p 179 (1980)

Gould considered the fossil record an ‘embarrassment’, for it almost never showed the process of evolution by natural selection that palaeontologists were supposed to be studying. Evolution proceeded jerkily (it showed a pattern of ‘punctuated equilibrium’), even when the inference that one species had descended directly or indirectly from another was reasonable enough.

The fossil record is punctuated by gaps. Some indicate that evolution happened rapidly and abruptly, not via a series of innumerable tiny steps. Others indicate that evolution did not happen at all: for example, that the trilobites had always been distinct from other kinds of animal. Either way, far from being the historical proof of Darwin’s theory, the fossil record proves to be emphatically at odds with it.

See also:
Antediluvian flora and fauna
Evolution? What evolution?