=Generalization and Specialization.=--A few terms in common use may receive a moment's discussion. A type or group is said to be specialized when it has a relatively large number of peculiarities or when some one peculiarity is carried to an extreme. A sculpin is a specialized fish having many unusual phases of development, as is also a swordfish, which has a highly peculiar structure in the snout. A generalized type is one with fewer peculiarities, as the herring in comparison with the sculpin. In the process of evolution generalized types usually give place to specialized ones. Generalized types are therefore as a rule archaic types. The terms high and low are also relative, a high type being one with varied structure and functions. Low types may be primitively generalized, as the lancelet in comparison with all other fishes, or the herring in comparison with the perch, or they may be due to degradation, a loss of structures which have been elaborately specialized in their ancestry. The sea-snail (_Liparis_), an ally of the sculpin, with scales lost and fins deteriorated is an example of a low type which is specialized as well as degraded. =High and Low Forms.=--In the earlier history of ichthyology much confusion resulted from the misconception of the terms "high" and "low." Because sharks appeared earlier than bony fishes, it was assumed that they should be lower than any of their subsequent descendants. That the brain and muscular system in sharks was more highly developed than in most bony fishes seemed also certain. Therefore it was thought that the teleost series could not have had a common origin with the series of sharks. It is now understood that evolution means chiefly adaptation. The teleost is adapted to its mode of life, and to that end it is specialized in fin and skeleton rather than in brain and nerves. All degeneration is associated with specialization. The degeneration of the blindfish is a specialization for better adaptation to life in the darkness of caves; the degeneration of the deep-sea fish meets the demands of the depths, the degeneration of the globefish means the sinking of one line of functions in the extension of some other.