Sentence:  Although Miller noted in 1907 that a <WSD>difference</WSD> in the pleural blood supply existed between animals , nowhere in his published works is it found that he did a comparative study of the intrapulmonary features of various mammalian lungs other than in the dog and cat ( Miller , ' 13 ; ' 25 ) . 
Meaning of the word marked with <WSD> tags:  a variation that deviates from the standard or norm