|Kew Gardens Palm House, public domain|
But the degree of adaptation of species to the climates under which they live is often overrated. We may infer this from our frequent inability to predict whether or not an imported plant will endure our climate, and from the number of plants and animals brought from warmer countries which here enjoy good health. We have reason to believe that species in a state of nature are limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates. But whether or not the adaptation be generally very close, we have evidence, in the case of some few plants, of their becoming, to a certain extent, naturally habituated to different temperatures, or becoming acclimatised: thus the pines and rhododendrons, raised from seed collected by Dr Hooker from trees growing at different heights on the Himalaya were found in this country to possess different constitutional powers of resisting cold. [emphasis mine]So, plants -- or animals, for that matter -- aren't necessarily selected for a specific environment, but are simply 'selected for' the ability to succeed in their current environment. This is, really, all Darwin said and it is very different from the widespread idea that every species is fine-tuned (or, if a generalist, was selected 'for' not being fine-tuned). This is really just an invocation of an axiom--an assumption not by itself a fact--that everything has been ordained by selection.
On this view, the capacity of enduring the most different climates by man himself and by his domestic animals, and such facts as that former species of the elephant and rhinoceros were capable of enduring a glacial climate, whereas the living species are now all tropical or sub-tropical in their habits, ought not to be looked at as anomalies, but merely as examples of a very common flexibility of constitution, brought, under peculiar circumstances, into play.And, Darwin concludes,
On the whole, I think we may conclude that habit, use, and disuse, have, in some cases, played a considerable part in the modification of the constitution, and of the structure of various organs; but that the effects of use and disuse have often been largely combined with, and sometimes overmastered by, the natural selection of innate differences.Whether or not his discussion of use and disuse verges on Lamarckism, the point here is that Darwin himself underscored the importance of adaptability, or facultativeness, in response to changing environments. He didn't say so, but we do in our book, The Mermaid's Tale, that the ability to adapt is a fundamental principle of life, and ubiquitous as it is, it was probably selected early in the history of life, and is fundamental to success. Unfortunately, Darwin was human and in many other places he states that effectively all variation is screened by selection. So, like all of us, he was somewhat inconsistent.
But this should be a sober reminder that, while selective explanations for any trait might be appealing, the most successful species are often those that can respond to change, not those that have fine-tuned their adaptation to a specific niche. The latter may be doomed to relatively quick extinction.