Spencer), as in the Araneae and Opiliones, and, on the other hand, may terminate in simplification and degeneration, as in the Acari.
A survey of the vegetable kingdom indicates that evolution has proceeded, on the whole, from the simple to the complex; at the same time, as has been already mentioned, evidence of reduction or degeneration in common.
The variety of special developments of structure accompanying the atrophy of typical organs in the Opisthobranchia and general degeneration of organization is very great.
So, too, degeneration is not to be lightly assumed as the explanation of a simplicity of structure.
There is a very definite criterion of the simplicity due to degeneration, which can in most cases be applied.