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.
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, and adaptation among fishes is almost as often degradation as advance. The bony fish is adapted to its mode of life, and to that end it is specialized in fin and skeleton rather than in brain and nerves as compared with its ancestors. All degeneration is associated with specialization. The degeneration of the blind fish 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 globe fish means the sinking of one line of functions in the extension of some other.
Referring to his own work on the fossil fishes in the early forties, Professor Agassiz once said to the writer:
This statement leads us to consider what is meant by 'high' and 'low.' Undoubtedly the sharks are higher than the bony fishes in the sense of being nearer to the higher vertebrates. In brain, muscle, teeth and reproductive structures, they are also more highly developed. In all skeletal and cranial characters the sharks stand distinctly lower. But the essential fact, so far as evolution is concerned, is not that the sharks are high or low. They are in almost all respects distinctly generalized and primitive. The bony fishes are specialized in various ways through adaptation to the various modes of life they lead. Much of this specialization involves corresponding degeneration of organs whose functions have ceased to be important. As a broad proposition, it is not true that 'we had our highest fishes first,' for in a complete definition of 'high' and 'low,' the specialized perch or bass stands higher. But whether true or not, it does not touch the question of evolution which is throughout a process of adaptation to conditions of life.
In another essay. Dr. Coues has compared species of animals to "the twigs of a tree separated from the parent stem. We name and arrange them arbitrarily in default of a means of reconstructing the whole tree according to nature's ramifications."
