When we say "missing link," we invoke a metaphorical chain, a set of links that stretch far back in time. Each link represents a single species, a single variety of life. Because each link is connected to two other links, each is intimately connected to past and future forms. Break one link, and the pieces of the chain can be separated, and relationships lost. But find a lost link, and you can rebuild the chain, reconnect separated lengths. One potent reason for the attractiveness of this metaphor is that it allows for the drama of the quest, the search for that elusive missing link.
Charles Sullivan and Cameron Mcpherson Smith go on to explain in the May 2005 issue of Skeptical Inquirer that while the metaphor is seductive, it's mistaken:
But the metaphor is as misleading as it is attractive. The concept that each species is a link in a great chain of life forms was largely developed in the typological age of biology, when species “fixity” (the idea that species were unchanging) was the dominant paradigm. Both John Ray (1627-1705) and Carolus Linnaeus (1707-1797), the architects of biological classification (neither of whom believed in evolution), were concerned with describing the order of living species, an order they each believed was laid out by God (Ray suggested that the divinely specified function of biting insects was to plague the wicked).
But while the links of a chain are discrete, unchanging, and easily defined, groups of life forms are not. We generally define a species as some interbreeding group that cannot, or does not, productively breed with another group. But since species are not fixed (they change through time), it can be difficult to be sure where one species ends and another begins. For these reasons, many modern biologists prefer a continuum metaphor, in which shades of one life form grade into another. Life is not arranged as links, but as shades. The metaphorical chain is far less substantial than it sounds.
Thus the chain metaphor is wrong. It doesn’t accurately represent biology as we know it today, but as it was understood over four centuries ago. The myth persists because of convenience; it is easier to think of species as types, with discrete qualities, than as grades between one species and another. In school, we learn the specific characteristics of plants and animals; this alone is not a problem, except that we are not often exposed to the main ramification of evolution: that those characteristics will change through time.
So, our idea that there can even be such a thing as a “missing link” was created in an era of biological research which is long gone. It’s a concept which is no longer valid in our current understanding of the nature of life and evolution — but, as is so often the case with appealing concepts, it continues to live on, to structure people’s assumptions, and to influence how they think about evolution.
This is almost always an unfortunate situation, but it is especially unfortunate here because the concept of a “missing link” creates confusion and misunderstandings which creationists are able to exploit. Perhaps the creationists know that they misunderstanding things; more likely, though, is that their misunderstanding is completely accidental and, in fact, one of the reasons why they are creationists in the first place.
Life is more like a spectrum than a chain, but not a spectrum with discreet ends. It’s a spectrum in that there are gradual transitions from one species to another, all of whom are certainly linked together, but no discreet links in a chain which can be broken, repaired, or readily followed. The more people understand this sort of thing, the better equipped they will be to understand evolution as a whole — not to mention the entire field of biology itself.