On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 14 : Mutual Affinities of Organic Beings | Summary

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Summary

Darwin dedicates this chapter to describing the current classification system for organisms, along with its inherent issues. He starts by defining "natural systems" but notes there is no clear definition for what a "natural system" is. In many cases what is meant by the term is the "plan of the Creator." He argues by organizing organisms by this "plan," nothing is "added to our knowledge." In other words organizing organisms this way serves no purpose in understanding the natural world. Instead, Darwin proposes classifying organisms based on ancestral descent. He sees this as the most obvious form of classification when considered as part of the theory of natural selection.

Darwin goes on to describe problems inherent with the current classification system, specifically with determining important versus non-important characteristics for classification. He relates this to variations in adult characteristics, as well as characteristics in the embryotic stage. Darwin argues the current classification system seems "arbitrary." He suggests the system should instead be genealogical in terms of the arrangement of larger groups and the degree of difference used to determine different genera and species. However, he notes analogous features can cause issues in determining actual genealogical relationships in contrast with homologous features that can help show ancestral descent. He also notes organs on an organism that bear the "stamp of inutility" can also be used to show previous ancestral connections between species or genera.

Darwin explicitly notes the work of Ernst Haeckel in developing phylogenies, proclaiming it as the future of classification systems. Darwin believes this system of classification should be adopted at the exclusion of all others. He ends the chapter with the following statement: "Species, genera and families ... are all descended, each within its own class or group, from common parents."

Analysis

Up to this point, the method of organism classification within science had been fairly arbitrary. It had been over a century since Carl Linnaeus had first published Systema Naturae in 1735. Yet, while the method for naming species and genera had been in use, the method for classifying these organisms had yet to be developed. Darwin's main argument in this chapter is the current system of classification—based on distinguishing between organisms that have been intelligently created versus those that had not—does not provide a useful system of classification. In other words it does not reveal anything about the natural world. Darwin's proposed classification system—based on the phylogenies of Ernst Haeckel—is grounded in the theory of natural selection. He argues it would be a more useful system of classification because it would explain the history of descent. This phylogenetic system is still in use today.

However, Darwin suggested this system of classification should be based on physical similarities of adults and embryotic stages. In contrast, most of the phylogenetic work today is based on genetic groupings of monophyletic organisms—organisms descended from a common ancestor. This work began in the second half of the 20th century and has since replaced phylogenies based on physical characteristics alone. However, Darwin's recommendation for the grouping of organisms based on these traits represents the first forays into phylogenetic groupings of organisms that inform much of biological and ecological work today.

Darwin's final statement—all organisms are descended from a common ancestor—is the logical extension of the theory of natural selection and reveals the continued insight and relevance of Darwin's thought process and work. Today, the modern definition of species is dependent on the idea of common ancestry.

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