Darwin's On the Origin of Species: A Modern Rendition

5. Variation

82 In earlier chapters I sometimes implied that variations are due to chance. Of course this is completely incorrect, but it illustrates our ignorance of the causes of variation. Some authors believe that the reproductive system functions in creating offspring that are similar to the parents and in generating individual differences or slight structural deviations, but the great frequency of variability and monstrosity under domestication suggests that structural deviations somehow result from the environment endured by parents and their ancestors over several generations. I remark in chapter 1 that the reproductive system is particularly susceptible to environmental changes. (Proving this requires a long catalog of facts that cannot fit here.) The varying and plastic condition of offspring results mainly from the functional disturbanceofthereproductivesysteminparents,withthereproductive elements seemingly affected before fertilization.1 It is unknown why a disturbedreproductivesystemshouldcausethisorthatpartofanorganism to vary more than usual. We can nevertheless occasionally catch hints and recognize that there must be some cause for each structural deviation, however slight. Howmuchdirecteffectclimate,food,andotherexternalfactorshave on an organism is uncertain–probably very little in animals and more in plants. Such direct influences could not have produced the many strikingandcomplexstructuralcoadaptationsamongorganismsthroughout 1. In “sporting” plants, which produce highly variable offspring, the buds are affected (in their earliest condition buds appear essentially the same as ovules). VARIATION 5 Variation 83 nature. Climate, food, and so on have some minor influence: E. Forbes confidently asserts that shelled mollusks living at their southern limit, or when living in shallow water, have more brightly colored shells than those of the same species farther north or from greater depths. Gould believes that birds of a given species are more brightly colored under a clear atmosphere than when living on islands or near the coast. Wollaston is convinced that insects’ residence near the sea affects their colors. Moquin-Tandon gives a list of otherwise non-fleshy plants that develop fleshy leaves when grown near the sea. Other such cases could be cited. When varieties of one species range into the habitats of other species , they often acquire some of the natives’ characteristics. This agrees with the concept that all species are simply well-defined and permanent varieties, illustrated by the above examples. A person who believes in the individual creation of each species would have to admit that one shell was created with bright colors for a warm sea but that another shell became bright-colored by variation when it ranged into warmer or shallower waters. When a variation is even slightly useful to its owner, we cannot tell howmuchofthatvariationresultsfromcumulativenaturalselectionand how much comes from direct environmental effects. Fur dealers know that animals of a given species have better and thicker fur if they live in a severe climate, but how much of this results from the warmest-clad individuals being preserved over many generations, and how much from the direct action of climate? It does seem that climate has some direct influence on the hair of domestic quadrupeds. There are examples of a single variety being produced under very different conditions and of different varieties of a single species being produced under the same conditions. This demonstrates how indirectly the environment acts. Naturalists know many examples of species that breed true, or do not vary at all, despite very contrasting climates. Such considerations suggest that little weight be given to direct environmental action. As already remarked, the environment plays an important and indirect role by affecting the reproductive system, thereby inducing variability. Natural selection, then, accumulates profitable variations, however slight, until they become appreciable. 84 Darwin’s On the Origin of Species Based on observations described in chapter 1, extensive use of certain body parts in domestic animals strengthens and enlarges them, while disuse diminishes them; such modifications are inherited.2 There are no standards of comparison in the wild by which to judge the effects of use and disuse, because the parental forms are unknown, but many structures can be explained by the effects of disuse. Professor Owen has remarked that flightless birds are a great anomaly in the wild, yet several such birds do exist. The logger-headed duck of South America can only flap along the surface of the water with its wings, which are in nearly the same condition as those of the domestic Aylesbury duck. Large groundfeeding birds rarely take flight except to escape danger. The nearly wingless condition of birds inhabiting, or having recently inhabited, oceanic islands that are free of...