Societies, like species, are in competition with each other, and thus, like species, are continually evolving into forms better suited to that struggle. Of course when I say “society” there are two things I might mean. Usually when I use the word I mean a number of individuals who interact in a structured way and share a common set of values. Thus when I exhort people to do what is best for society I mean to say that they should do what is best for this group of people. Here, however, I am considering society in the abstract sense, as a set of values that governs the way a group of people structure their lives and interactions. In this sense society corresponds roughly to species. Societies and species of course do not directly compete with each other; the idea doesn’t even make sense, since they are abstractions and not physical entities, and are thus unable to struggle with each other. However, just as species are instantiated in individuals, so are societies are instantiated in groups of individuals. And these groups of individuals may struggle with each other, with their success determined in part by the strength of the abstract society that they are instantiating.
It is hard to say exactly how one society is doing in comparison to another. We might be tempted to compare the number of people composing groups that fall under this abstract society, or the number of distinct groups falling under it. Such measures are like comparing the success of species by arguing that fleas are more successful than dogs because they outnumber them, and the bacteria that live in the gut of the flea more successful yet. Such a measurement is relatively easy to make, but it doesn’t necessarily reveal how well the species, as an abstract, is doing in comparison to other abstract species. Fleas could be argued to be more successful than dogs, but the bacteria specially adapted to live in side fleas are certainly less successful than the fleas themselves. A better measure is to compare how long the abstract species or society, or one of its descendant species or societies, survives (meaning how long there is something that instantiates it). Thus the success of a species or society is not measured by how numerous it is, but how many variations arise, and whether these variations can survive changes in the environment. Thus a society which is instantiated by a single group of people may be more successful than its more numerous contemporaries if its structure allows it to better to adapt to changing times, and thus to outlast them.
With that said there are basically three ways for societies to compete: militarily, economically, and socially. Military competition tends to favor the societies that are most efficiently able to use their resources, and which are most technologically advanced. Military competition allows one society to out compete another by either by eliminating all the individuals who previously composed groups forming different societies, or by forcefully imposing a different social order on the conquered group. However, military competition has a chance of going both ways, meaning that victory is never completely assured. Thus societies that tend to avoid military competition are probably going to be more successful in the long run. Moreover, if military competition was central to the success of some society then we would expect many groups to structure themselves according to those social principles, thus leading to a state of near constant conflict, or a situation in which almost everyone belongs to a single group of people, also a fragile state of affairs. Thus economic competition is a better strategy then military competition. To compete economically means that both societies are attempt to increase their share of the available resources, which means reducing the share given to other groups. And this can be accomplished by being better at turning those resources into products, thus leading people from the other society to desire them, making that society wealthier, and thus able to control more resources. Economic competition isn’t as self-destructive as military competition has the possibility to be; even if every society is competing economically that doesn’t necessarily lead to disaster. But the problem with economic competition is that there is no ability to eliminate rivals as there is with military competition. At best all you can do is reduce them to poverty. Thus economic competition must be supplemented with social competition. Social competition is the most passive form of competition, and one that has no biological counterpart. Social competition is a process of eliminating a society by leading the groups that instantiate it to change their society, or by depopulating them because their members leave to join a group with a different society. This is the best kind of competition, at least from the perspective of individuals, because it encourages society to serve the needs and wants of the individuals that compose as best it can, in order to attract more people.
Of course if societies can compete socially there will be defenses against such competition. Forbidding immigration is not one of those defenses, however. In fact forbidding immigration is effectively to cripple the ability of your own society to compete socially. Naturally some people worry that immigrants will import their own customs, and thus undermine the existing society. It is true that immigrants may have an effect on the dominant culture, but they are unlikely to influence the social structure, because traditions and customs are simply window dressings for the values of society; the same traditions can exist in a number of different societies. What is more important are the laws, and it is rare for individuals to emigrate with the idea of disobeying the laws of their new home. The real defense mechanism against social competition is the dogmatic attitude that society’s existing values and structure is perfect (see the rabid pro-capitalism, anti-communism attitudes during the cold war). But while such attitudes may be good for society they are bad for individuals. Don’t we want to live in the best society possible? If that is the case then a mind closed to alternatives isn’t helping us.