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24 June 2019

The Origin of Species: Darwin’s Corrections

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In 1859, Charles Darwin published his culminating work On the Origin of Species, which immediately received many different objections and critiques from various angles – both scientific and religious as well as political and sociological, among others. In response to these objections, Darwin edited his work on at least five occasions. In the sixth and final edition in 1872,  he touched up certain parts and concepts that had been called into question. Do his responses to the critiques remain valid today?

THE RELIGIOUS ISSUE

In the first edition of On the Origin of Species, Darwin explained how the different species of living organisms were created through natural selection from a common ancestor (1). Thus, he also clearly positioned himself against the possibility that the species were independently created. This was the motive for the enormous criticism from different religious sectors as it did away with the idea of a creator’s involvement in this process. The most controversial part was that the last sentence of the summary of his piece, which did not include such a creator anywhere:

“There is grandeur in this view of life…whilst this planet has gone cycling…from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

In the second edition in 1860, just a few months after the first – Darwin made two tweaks in terms of religion. First, he mentions a creator in the previous sentence, something that remained in subsequent editions, which could be interpreted as an attempt to reassure his critics.

In the second version of On the Origin of Species, Darwin deliberately mentions a “creator”. Image: Darwin Online

And above all, in the final pages he added several sentences in which he maintains: “I see no good reasons why the views given in this volume should shock the religious views of anyone.” To justify this argument, he turned to a famous author and clergyman (Charles Kingsley): “I have gradually learnt to see that it is just as noble a conception of Deity, to believe that he created primal forms capable of self development into all forms…”

GENETIC VARIATIONS AND INHERITANCE: AN ENRICHING CRITIQUE

In the third edition in 1861, Darwin included a preface on the historical background of his theory, and only made minor corrections in the fourth (1866). Meanwhile, in the fifth edition (1869), he made several additions to try to address a significant controversy over his theory, which was raised in an article published in 1867 by the Scottish engineer Fleeming Jenkin in the North British Review (2).

La refutación de Fleeming Jenkin (en la imagen) hizo que Darwin enmendara su obra/ Image: wikimedia

Fleming Jenkin’s rebuttal (in the image) led Darwin to amend his work. In the first four editions, Darwin had assumed that there were two types of variations on which natural selection acts: small and frequent variations, and large and rare variations. Jenkins argued that if the variations favored by natural selection are large in scope, they are very unlikely to persist in succeeding generations. He used the following rationale to defend this idea:

  • These “rare” individuals that carry the favorable variation would have to reproduce with other individuals without this variation, which are the most abundant.
  • Most of the offspring of these combinations would not have the favorable variation because both Jenkin and Darwin accepted the idea that prevailed at the time that children receive a mix of their parents’ inheritances.   
  • As a result, the likelihood that the favorable variation would persist in the next generation would be low, and would further decrease in subsequent generations.

Given this controversy, in Chapter 4 of the fifth edition Darwin gives less importance to these large and rare variations and focuses more on the small and frequent variations in which the combined inheritance does not pose a problem. If these small and frequent variations could be established in populations, they would be recurrent, and the individuals carrying them could recognize each other to reproduce. Ultimately, this difficulty allowed Darwin to reaffirm his idea that evolution occurs gradually and not in “leaps” – something that remains accepted today, although with a different idea regarding the inheritance of variations – both small and also large, which came from Mendelism.   

NATURAL SELECTION AND EVOLUTION

The biggest change that Darwin made in the sixth edition (1872) was the addition of a chapter, Chapter 7, in which he attempted to respond to objections raised by the zoologist  St. George Jackson Mivart regarding the role of natural selection in evolution.

St. George Jackson Mivart’s objections led Darwin to consider his most significant change. / Image: wikimedia

In the first five editions of his work, Darwin maintained that the origin of complex functional structures, such as wings or eyes, had come about as a result of selection from simple, even rudimentary structures.  Mivart published a text On the Genesis of Species (3) in which he accepted the idea of evolution, but was against natural selection being its most important mechanism.

Mivart’s main difficulty with Darwin’s theory was summarized in the title of the first chapter of his book: The Incompetency of “Natural Selection” to Account for the Incipient Stages of Useful Structures. Mivart argues that it is very difficult to explain the onset of structures like wings through natural selection as it would be hard for them to work for flight in the early stages, and the same would apply to other structures like eyes and the ability to see. Mivart suggests that these structures would have appeared already developed by significant modifications to the development patterns present in the organisms’ ancestors.

In the sixth edition, Darwin continued to maintain his position that eyes had emerged from very simple structures through natural selection, improving the ability to see.  But for wings (in the previously mentioned Chapter 7 and also in Chapter 5, which he rewrote for the sixth edition), he puts the emphasis on an idea that was not sufficiently developed in prior editions: these structures that we now see with a certain function could have emerged and been selected in their early stages for a different function for which they did not need to be very developed. For example, the “proto-wings” of insects or birds could have had a thermoregulation function and when they reached a certain size,  started to have a function designed to allow the act of flying.

Darwin maintained that wings are an evolutionary development of a trait previously designed for thermoregulation.

Over time, different findings and experiments have demonstrated that Darwin’s ideas on this issue were more accurate than Mivart’s. For example, experimental studies carried out in the 20th Century have revealed how the transition of wings from thermoregulation to flight could have taken place. Specifically, this is something that could be related to the increase in size of the body and wings of organisms: the bigger organisms and proto-wings got, the more quickly this change could occur (4). And even for other structures as complex as the eyes of octopi, eagles or humans, for which even Darwin had his doubts that they could have been formed through natural selection from simple eyes like the eyespots on protists like Euglena.

It has also been demonstrated more recently that the 40 different kinds of eyes that exist in nature – from the simplest to the most complex – have a common genetic makeup on which natural selection has acted. This common genetic network is comprised of genes like Pax6 that condition the formation of the eye in a certain cephalic area by acting on other genes. They also share other genes that are common to all organisms which control the synthesis of the pigments and structures that play a role in sight (5).

CONCLUSION

It is clear from this article that the subsequent editions of On the Origin of Species responded to the most important critiques and objections made, thus enriching its theory in the process. Therefore, the sixth edition of 1872 – and not the first edition of 1859 – is currently considered the definitive and most important edition,  the canon to analyze his work. Furthermore, it is also clear that although he never modified his theories in response to the scientific criticism, he did adjust them, getting it right in aspects such as the type of variations that are important for evolution. Darwin himself had to “evolve” his theory, finding solutions that were also accurate for problems such as the initial stages of certain structural functions of living organisms (wings).

Additional objections were later made to Darwin’s theory by other professionals – geneticists, paleontologists, molecular biologists, ecologists, etc. – all trying to remedy, polish and complete his work, but leaving its essence in tact.

BIBLIOGRAPHY

  1. The six editions of On the Origin of Species can be found on the web site DARWIN ON LINE.
  2. Jenkin, F. 1867.The Origin of Species.North British Review.46:277-318.
  3. Mivart,St.G.J. 1871. The Genesis of Species..
  4. Kingsolver,J.C. , Koehl, MAR, 1985. Aerodynamics, thermoregulation and the evolution of insect wings. Evolution, 39(3):488-504.
  5. Gehring, W.J. 2004.New Perspectives on Eye Development and the evolution of eyes and photoreceptors. Journal of Heredity. 96(3): 175-184.

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