(5) A crowd of difficulties (pages 65-73)
(6) Throughout time (pages 73-82)
(5) A crowd of difficulties (pages 65-73)
In Chapters 5-8 Origin sweeps forward through a wide range of biological topics, but first it addresses some of the serious objections to Darwin’s theory:
Q: If there were intermediate forms or ‘missing links’, where are they now? In the long, slow process of evolution, the intermediate forms have gone extinct. We can’t even find most of them in fossil form, because the fossil record is imperfect.
Q: How do we explain animals that bridge two ways of living, like flying mammals, flightless birds, or the ancient proto-seal which moved from land to sea? (see Beasts, p. 66) To envision these transitions, it is also helpful to think of a single organ (such as the bones of the hand) with a structure which can be repurposed – perhaps not perfectly, but just enough to allow its owner to be a bit better adapted to a new environment.
Q: How do we explain useless organs, like the human appendix? In the long process of evolution, organs of little apparent purpose today were originally of high importance to progenitors. Herein is an argument against ‘special creation’, for why would a divine architect deliberately design such purposeless organs?
Q: How do we explain organs of extreme perfection? Darwin’s chosen example is the eye. Its exquisite design makes an especially compelling case for special creation, since it is highly unlikely that each part could have evolved at the same time. But such a complex organ could be formed by small, transitional gradations. Looking across the whole animal kingdom, we find a range of photosensitive organs. Imagine the eye, Darwin writes, beginning simply as a nerve that becomes sensitive to light. An organism does not necessarily need a perfect eye to benefit from sensitivity to light. There is no need to assume that the eye would have to become a complex structure before it became functional. Complexity arises through a process that is like climbing a long, winding staircase one step at a time.
Q: How do we explain animal instincts? Take the perfect geometry of the honeycomb, constructed by little bees of low intelligence. How do they know when the comb is done correctly? Natural selection would favor the swarm which wasted the least honey in the secretion of wax – and incremental improvement in the construction of the comb would yield incremental advantages to the bees that made it. The same type of reasoning can be applied to all the instincts of the social insects.
Q: How do we reconcile all this suffering to the idea of a loving God? Darwin’s discussion of the evolution of instincts ends with a sobering afterthought. He has been sensitive all along to the high cost of suffering paid by sensate animals. In ‘special creation’, all this is according to divine design. Darwin finds it much more satisfactory to look upon these instinctual behavior “not as specially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die.” It is somewhat of a relief to think of this law of nature rather than attributing these instincts to the direct plan of a loving God.
Thus difficulties which could have brought down the theory serve instead to give it more credibility. In the process, the explanatory power of the theory has been displayed.
And now, with major difficulties addressed, Darwin takes his view of life deep into time and wide across space.
(6) Throughout time (pages 73-82)
The grand vision of universal descent requires a grand vision of time. Darwin did not know the age of the earth in absolute years, but he did know that the earth is very, very old. When we consider this vast span of time, we begin to understand that there has been enough time for natural selection to work.
By the mid-19th century scientific knowledge of the earth and its deposits was booming. Paleontologists who studied fossils embedded in rock layers noted how species appeared, expanded, and disappeared. Once they made the connection between the sequential age of rock layers and the fossils they held, they started to construct a history of life, arranging species in order of chronological appearance. Drawing on this knowledge, Darwin put his theory to work to interpret the past.
In the mid-19th century there were two competing theories about how the earth’s landscape was formed. Some argued that landscapes were shaped quickly due to violent calamities, such as a flood. Others, like Charles Lyell in Principles of Geology, argued instead that the earth’s features were formed gradually by forces still active today. (Darwin loved Lyell’s book, having read it on the Beagle and finding evidence for its position during his travels.) Now Darwin used Lyell’s insights to argue that the earth is ancient beyond measure.
But why doesn’t the geologic record show a sequence of fossils? Most living organisms never become fossils, because of the special conditions required. The most likely time for fossils to form is during the slow sinking of a region, when sediment washes in from streams and infiltrates organisms as they die. The least likely time is when the land is rising upward, leaving no place for sediment to amass – and exposing existing fossils to a gauntlet of destructive processes. Thus periods of uplift are more conducive to the formation of new species, as the environment opens up new niches for varieties to branch out and colonize – but during these periods there will generally be a blank in the geologic record.
And so, few fossils remain in the geologic record. Of this multi-volume history of life we have only the last volume; in this volume there are left only a few short chapters; in each chapter, only a few pages here and there; on each page, only a few lines, in each line, only a few words…. Without forcing the evidence, Origin sets out to read the ‘words’ that are still available, and to decode the blank spaces.
Darwin argues that natural selection accounts for the patterns we find in the geological record. Over the grand sweep of time species slowly adapt to new conditions of life, are selected for advantageous changes, and in turn produce offspring who inherit the new characteristics. The general rule seems to be a gradual increase in a species’ number until the group reaches its maximum; then gradual decrease and final disappearance, with rare excepts. This accords more with natural selection than some supernatural agency.
Time and again Origin emphasizes the finality of the death of a species. Species once lost do not reappear. Modern researchers estimate that a complete inventory of all the species that have ever lived would number in the billions. The notion that all inhabitants of the earth have periodically been swept away by catastrophes does not hold much appeal. Instead, the study of rock formations reveals that species gradually disappear. A species that does not adapt is reduced to fewer numbers, and rarity then leads to the probability of extinction.
Darwin called the relationship between dead and living species “the law of the succession of types” or “this wonderful relationship in the same continent between the dead and the living.” He saw living species as the budding twigs at the top of the tree of life, and fossils as the underlying branches. The longer back in time we go, the closer species approach one another in structure and function.
By tracking the history of life back into deep time, Origin demonstrates the strength of the argument that all forms of life, ancient, recent, and now living, unroll through the eons as one grand natural system, linked by generation. While demonstrating the explanatory power of the theory, Darwin’s geological chapters also alert the mind to marvels in the history of life:
- enormous intervals of time
- irreplaceable extinctions
- incalculable numbers of generations going forward
- profound affinities between the living and the dead
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