The original maize plant was indigenous to the Western Hemisphere.  The plants were weedlike and only through the efforts of early breeders were cultivated crop species developed.  The crop cultivated by early breeders, like the crop today, could be wind pollinated.  The physical traits of maize are such that wind pollination results in self-pollination or cross-pollination between plants.  Each plant has a separate male and female flower that contributes to pollination, the tassel and ear, respectively.  Natural pollination occurs when wind transfers pollen from tassel to the silks on the corn ears.  This type of pollination has contributed to the wide variation of maize varieties present in the Western Hemisphere. 
The development of a planned breeding program for maize only occurred in the last century.  A large part of the development of the maize product into a profitable agricultural crop was due to the work done by land grant colleges.  Originally, maize was an open pollinated variety having heterogeneous genotypes.  The maize farmer selected uniform ears from the yield of these genotypes and preserved them for planting the next season.  The result was a field of maize plants that were segregating for a variety of traits.  This type of maize selection led to, at most, incremental increases in seed yield. 
Large increases in seed yield were due to the work done by land grant colleges that resulted in the development of numerous hybrid corn varieties in planned breeding programs.  Hybrids were developed by selecting corn lines and selfing these lines for several generations to develop homozygous pure inbred lines.  One selected inbred line was crossed with another selected inbred line to produce hybrid progeny (F1).  The resulting hybrids, due to heterosis, are robust and vigorous plants.  Inbreds on the other hand are mostly homozygous.  This homozygosity renders the inbred lines less vigorous.  Inbred seed can be difficult to produce since the inbreeding process in corn lines decreases the vigor.  However, when two inbred lines are crossed, the hybrid plant evidences greatly increased vigor and seed yield compared to open pollinated, segregating maize plants.  An important consequence of the homozygosity and the homogenity of the inbred maize lines is that all hybrid seed produced from any cross of two such elite lines will be the same hybrid seed and make the same hybrid plant.  Thus the use of inbreds makes hybrid seed which can be reproduced readily.  The hybrid plant in contrast does not produce hybrid seed that is readily reproducible.  The seed on a hybrid plant is segregating for traits. 
The ultimate objective of the commercial maize seed companies is to produce high yielding, agronomically sound plants that perform well in certain regions or areas of the Corn Belt.  To produce these types of hybrids, the companies must develop inbreds, which carry needed traits into the hybrid combination.  Hybrids are not often uniformly adapted for the entire Corn Belt, but most often are specifically adapted for regions of the Corn Belt.  Northern regions of the Corn Belt require shorter season hybrids than do southern regions of the Corn Belt.  Hybrids that grow well in Colorado and Nebraska soils may not flourish in richer Illinois and Iowa soil.  Thus, a variety of major agronomic traits are important in hybrid combination for the various Corn Belt regions, and have an impact on hybrid performance. 
Inbred line development and hybrid testing have been emphasized in the past half-century in commercial maize production as a means to increase hybrid performance.  Inbred development is usually done by pedigree selection.  Pedigree selection can be selection in an F2 population produced from a planned cross of two genotypes (often elite inbred lines), or selection of progeny of synthetic varieties, open pollinated, composite, or backcrossed populations.  This type of selection is effective for highly inheritable traits, but other traits, for example, yield requires replicated test crosses at a variety of stages for accurate selection. 
Maize breeders select for a variety of traits in inbreds that impact hybrid performance along with selecting for acceptable parental traits.  Such traits include: yield potential in hybrid combination; dry down; maturity; grain moisture at harvest; greensnap; resistance to root lodging; resistance to stalk lodging; grain quality; disease and insect resistance; ear and plant height.  Additionally, Hybrid performance will differ in different soil types such as low levels of organic matter, clay, sand, black, high pH, low pH; or in different environments such as wet environments, drought environments, and no tillage conditions.  These traits appear to be governed by a complex genetic system that makes selection and breeding of an inbred line extremely difficult.  Even if an inbred in hybrid combination has excellent yield (a desired characteristic), it may not be useful because it fails to have acceptable parental traits such as seed yield, seed size, pollen production, good silks, plant height, etc. 
To illustrate the difficulty of breeding and developing inbred lines, the following example is given.  Two inbreds compared for similarity of 29 traits differed significantly for 18 traits between the two lines.  If 18 simply inherited single gene traits were polymorphic with gene frequencies of 0.5 in the parental lines, and assuming independent segregation (as would essentially be the case if each trait resided on a different chromosome arm), then the specific combination of these traits as embodied in an inbred would only be expected to become fixed at a rate of one in 262,144 possible homozygous genetic combinations.  Selection of the specific inbred combination is also influenced by the specific selection environment on many of these 18 traits which makes the probability of obtaining this one inbred even more remote.  In addition, most traits in the corn genome are regrettably not single dominant genes but are multi-genetic with additive gene action not dominant gene action.  Thus, the general procedure of producing a non segregating F1 generation and self pollinating to produce a F2 generation that segregates for traits and selecting progeny with the visual traits desired does not easily lead to a useful inbred.  Great care and breeder expertise must be used in selection of breeding material to continue to increase yield and the agronomics of inbreds and resultant commercial hybrids. 
Certain regions of the Corn Belt have specific difficulties that other regions may not have.  Thus the hybrids developed from the inbreds have to have traits that overcome or at least minimize these regional growing problems.  Examples of these problems include in the eastern corn belt Gray Leaf Spot, in the north cool temperatures during seedling emergence, in the Nebraska region CLN (corn Lethal necrosis and in the west soil that has excessively high pH levels.  The industry often targets inbreds that address these issues specifically forming niche products.  However, the aim of most large seed producers is to provide a number of traits to each inbred so that the corresponding hybrid can useful in a broader regions of the Corn Belt.  The new biotechnology techniques such as Microsatellites, RFLPs, RAPDs and the like have provided breeders with additional tools to accomplish these goals.