In the eighteenth century the French naturalist Jean Baptiste de Lamarck believed that an animal's use or disuse of an organ affected that organ's development in the animal's offspring. Lamarck claimed that the giraffe's long neck, for example, resulted from its ancestors stretching to reach distant leaves. But because biologists could find no genetic mechanism to make the transmission of environmentally induced adaptations seem plausible, they have long held that inheritance of acquired characteristics never occurs. Yet new research has uncovered numerous examples of the phenomenon.

In bacteria, for instance, enzymes synthesize and break down rigid cell walls as necessary to accommodate the bacteria's growth. But if an experimenter completely removes the cell wall from a bacterium, the process of wall synthesis and breakdown is disrupted, and the bacterium continues to grow—and multiply indefinitely—without walls. This inherited absence of cell walls in bacteria results from changes in the interactions among genes, without any attendant changes in the genes themselves.

A fundamentally different kind of environmentally induced heritable characteristic occurs when specific genes are added to or eliminated from an organism. For example, a certain virus introduces a gene into fruit flies that causes the flies to be vulnerable to carbon dioxide poisoning, and fruit flies infected with the virus will pass the gene to their offspring. But if infected flies are kept warm while they are producing eggs, the virus is eliminated from the eggs and the offspring are resistant to carbon dioxide. Similarly, if an Escherichia coli bacterium carrying a certain plasmid—a small ring of genetic material—comes into contact with an E. coli bacterium lacking the plasmid, the plasmid will enter the second bacterium and become part of its genetic makeup, which it then passes to its offspring. The case of the E. coli is especially noteworthy for its suggestion that inheritance of acquired characteristics may have helped to speed up evolution: for example, many complex cells may have first acquired the ability to carry out photosynthesis by coming into contact with a bacterium possessing the gene for that trait, an ability that normally would have taken eons to develop through random mutation and natural selection.

The new evidence suggests that genes can be divided into two groups. Most are inherited "vertically," from ancestors. Some, however, seem to have been acquired "horizontally," from viruses, plasmids, bacteria, or other environmental agents. The evidence even appears to show that genes can be transmitted horizontally between organisms that are considered to be unrelated: from bacteria to plants, for example, or from bacteria to yeast. Such horizontal transmission may well be the mechanism for inheritance of acquired characteristics that has long eluded biologists, and that may eventually prove Lamarck's hypothesis to be correct.