In prehistoric times brachiopods were one of the most abundant and diverse forms of life on Earth: more than 30,000 species of this chainlike creature have been cataloged from fossil records. Today brachiopods are not as numerous, and existing species are not well studied, partly because neither tile animal' s fleshy inner tissue flor its shell has any commercial value. Moreover, in contrast to the greater diversity of the extinct species, the approximately 300 known surviving species are relatively, uniform. in appearance. Many zoologists have interpreted this as a sign that the animal has been unable to compete successfully with other marine organisms in the evolutionary struggle. Several things, however, suggest that the conventional view needs revising. For example, the genus Lingula has an unbroken fossil record extending over more than half a billion years to the present. Thus, if longevity is any measure, brachiopods are the moat successful organisms extant. Further, recent studies suggest that diversity among species is a less important measure of evolutionary success than is the ability to withstand environmental change, such as when a layer of clay replaces sand on the ocean bottom. The relatively greater uniformity among the existing brachiopod species may offer greater protection from environmental change and hence may reflect highly successful adaptive behavior. The adaptive advantages of uniformity for brachiopods can be seen by considering specialization, a process that occurs as a result of prolonged colonization of a uniform. substrate. Those that can survive on many surfaces are called generalists, while those that can survive on a limited range of substrates are called specialists. One specialist species, for example, has valves weighted at the base, a characteristic that assures that the organism is properly positioned for feeding in mud and similar substrates other species secrete glue allowing them to survive on the face of underwater cliffs. The fossil record demonstrates that most brachiopod lineages have followed a trend toward increased specialization. However, during periods of environmental instability, when a particular substrate to which a specialist species has adapted is no longer available, the species quickly dies out. Generalist, on the other hand are not dependent on a particular substrate, and are thus less vulnerable to environmental change. One study of the fossil record revealed a mass extinction of brachiopods following a change in sedimentation from chalk to clay. Of the 35 brachiopod species found in the chalk, only 6 survived in the clay, all of them generalists. As long as enough generalist species are maintained, and studies of arctic and subarctic seas suggest that generalists are often dominant members of the marine communities there, it seems unlikely that the phylum is close to extinction. In the passage, the author is primarily concerned with______.