Objecting to the Evidence
Having introduced the basic concepts behind his theories, Darwin spends much of the book providing evidence and examining more specific processes through which his mechanisms work. At times he seem almost irritated that more attention has not been paid to the clues left by the natural world, for example: "Nature may be said to have taken pains to reveal, by rudimentary organs and by homologous structure, her scheme of modification, which it seems that we wilfully will not understand." However, in 1861, less than two years after the publication of Origin, in a session before the British Association for the Science, a critic claimed that Darwin's book was too theoretical and that he should have just "put his facts before us and let them rest. In a letter to his friend Henry Fawcett, who was in attendance in his defence, Darwin explained the proper relationship between facts and theory: "About thirty years ago there was much talk that geologists ought only to observe and not theorize; and I well remember someone saying that at this rate a man might as well go into a gravel-pit and count the pebbles and describe the colours. How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service!" This remains a central tenet through which science must operate: if scientific observations are to be of any use, they must be tested against a theory, hypothesis, or model.
This is not to say that Darwin was overly dogmatic about his theories - indeed as has been seen he was persuaded to change some of them - but he is sensible enough to admit areas of ignorance, and pre-empts his critics by including a chapter "Difficulties on Theory". Some of these he believes to be "so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgement, the greater number are only apparent, and those that are real are not, I think, fatal to my theory."
One of the most consistently troublesome problem that Darwin tackles is that of the lack of intermediate types: "why if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?" And why does the fossil record show evidence of the same? Darwin writes that, "the answer mainly lies in the record being incomparably less perfect than is generally maintained supposed I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a few lines. Each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparent abruptly changed forms of life, entombed in our consecutive, but widely separated, formations."
Whilst many more discoveries have been made since the time of Darwin, the fact does remain that what has been discovered often points towards the abrupt succession of fossil types. It would seem, as is maintained now by biologists such as Steven J. Gould (see Further Reading), that certain forms remain stable over long periods and are then in relatively short spaces of time succeeded by radically different forms. This is consistent with Darwin's geological metaphor: whilst erosion is gradual, occasionally a chunk of a cliff might topple into the sea. However, this idea of 'Punctuated Equilibria' is not universally accepted by the gradualist camp, and the exact mechanism through which the sudden changes take place is an area of continued controversy, particularly with regard to the viability of individuals harbouring the sudden leaps in genetic variation necessary for some of the proposed transformations - the "hopeful monsters" as Gould calls them. Beneath the specifics, however, it is important not to lose sight of the fact that, as Darwin perceived, viability of change is always in relation to the many facets of the environment in which organisms inhabit, and the relationship between these organisms. Natural Selection can be 'stabilising' as well as producing change and divergence. As for living 'intermediates', it is a process that will take place on a population rather than individual level. It is not the case of finding a single organism 'half-way' to becoming a new species, but rather that of populations within species becoming isolated (which can be by resource availability, and behavioural changes as well as just geographical separation) and diverging to the point that interbreeding with the original species is no longer a possibility. And Darwin recognises that many species are in fact intermediates in the vast spectra of organisms - giving examples of transitional behaviours, morphological features and habitats that exist from population to population, variety to variety, and species to species.
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