However, this is a generalization that does not apply in many specific instances.
There is no exception to this generalization.
It may be added that we have here no generalization imported from a higher level of culture, but an idea or blend of ideas familiar to primitive thought.
The details of the logical method on which its author laid the greatest stress have not been found of practical service; 5 yet the fundamental ideas on which the theory rested, the need for rejecting rash generalization, and the necessity for a critical analysis of experience, are as true and valuable now as they were then.
Having accepted these two conclusions, we formulate the generalization that tracheae can be independently acquired by various branches of Arthropod descent in adaptation to a terrestrial as opposed to an aquatic mode of life.