Sentence:  Yet it is not difficult to see that <WSD>Oölogy</WSD> promises to become an important auxiliary both in aiding to determine natural divisions, and to enable us to decide in regard to varieties the specific identity of which is in doubt. Let us take an instance. [...] Without looking at all to the external structure of these birds, <WSD>Oölogy</WSD> would seem to indicate that the Cat-Bird, though closely allied to the true Mocking-Bird, is at least one step, and that the Brown Thrush is even farther, removed. 
Meaning of the word marked with <WSD> tags:  The study of birds' eggs. (from 19th c.)