The Origin of Species: Chapter 5 Summary & Analysis

Darwin began laying out his plan to explain in more detail how variation arose in species. He acknowledged that it was difficult to tell precisely what role factors like climate or food supply play in any given situation. Ultimately, he believed that the conditions of life not only cause variations but also natural selection.

Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection. Many of Darwin’s peers and predecessors wondered about why many creatures have “deficient” parts, such as beetles with wings that can’t fly. Darwin believed such features could be explained through natural selection, which involved gradual selection based on lack of use. This is why some subterranean animals lack proper eyes, whether in the Old World or the New.

Acclimitisation. Plants inherit habits like flowering periods and sleep schedules through heredity. While nature usually limits the range of organisms, sometimes, plants can become acclimated to new regions and thrive, even better than in their original home. How much a plant or animal species can get acclimated is based on its habits. Habits include use and disuse of various parts, and they play a role in modifications through natural selection.

Correlated Variation. When some parts of an organism are modified, the change can have a related effect on other parts. For example, changes in embryos or larvae can affect a mature adult animal. Another example is how the shape of the pelvis bones in a bird seem to be correlated to the shape of its kidneys.

Compensation and Economy of Growth. Darwin examined the idea of some of his predecessors that when nature gives to one area of an organism, it takes away from another. It is difficult, for example, to fatten a cow and have it create a lot of milk at the same time. Darwin expanded on the idea, suggesting that natural selection is always trying to organize things more efficiently. This means getting rid of characteristics or habits that become unnecessary.

Multiple, Rudimentary, and Lowly-organised Structures are Variable. For both varieties and species, “rudimentary parts” (like the vertebrae on a snake) are likely to be variable, since individually, they are not especially useful.

A Part developed in any Species in an extraordinary degree or manner, in comparison with the same part in allied Species, tends to be highly variable. Darwin noted that the most developed parts and organs in a species tend to be the most important and therefore the most subject to variation. He looked at the example of the wing of the bat, which is abnormal among mammals, and concluded that it was not a good example of this principle because it must have existed for a long time in its current state. It is only more recent highly developed parts that show an increase in variability.

Specific Characters more Variable than Generic Characters. Darwin believed that species-level characteristics are more variable than genus-level ones. He used the example of a large genus of plants with some blue flowers and some red flowers and how variation here would be more expected than in a genus where all plants within it were blue.

Secondary Sexual Characters Variable. Following the opinion of most other naturalists, Darwin believed that secondary sexual characteristics are highly variable. For example, in birds where males have elaborate feather patterns to attract females, there is a lot of variation in the males’ feathers.

Distinct Species present analogous Variations, so that a Variety of one Species often assumes a Character Proper to an allied Species, or reverts to some of the Characters of an early Progenitor. Darwin noted that domestic pigeons, even in countries far apart, sometimes develop variations more typically found in pigeons from other parts of the world. This seems to be the result of them having a common parent species. Even characteristics that have been lost for many generations can suddenly reappear in offspring.

Darwin also looked at the example of horses. He found that throughout the horse genus, there was a tendency for offspring in different species to develop some form of stripes. He believed this was evidence that each species of horse was not “independently created,” but that in fact all modern creatures in the horse genus once had an ancestor that was striped like a zebra (although it may have had a different form).

Summary. Darwin admitted that he and other naturalists were still ignorant about many of the laws surrounding variations. Nevertheless, he believed there was enough evidence to draw some conclusions based on the patterns visible in samples from the species and genera that existed in his day.