A sentence fragment is missing a subject, a verb, or both, but is punctuated as if it were a complete sentence.
By virtue of their prevalence alone. It is clear that
mood disorders do not necessarily breed genius.
[By virtue of their prevalence alone contains neither a subject nor
To repair a sentence fragment, add the missing element (subject or verb), or merge the fragment with a clause that does contain a subject and a verb.
By virtue of their prevalence alone, it is clear that mood disorders do not
necessarily breed genius.
--"Manic-Depressive Illness and Creativity," Scientific American
When you edit your documents, check to be sure that you have not used any sentence fragments.