1. ^[https://books.google.com/books?id=xsi6QcQPJGkC&pg=PA218&lpg=PA218&dq=%22progressive+reduction+of+variability%22&source=bl&ots=QtKzpDJH6U&sig=omzAdF6htNfQSls7jaxsaWjQI-M&hl=en&sa=X&ei=FWJBU8n8KvTHsATLpICIDw&redir_esc=y#v=onepage&q=%22progressive%20reduction%20of%20variability%22&f=false Phylogenetic Systematics], by Willi Hennig (translated from German by D. Dwight Davis and Rainer Zangerl); originally published in German in 1966; published in 1979 by University of Illinois Press (via Google Books)
2. ^{{cite journal|first= Nigel C.|last= Hughes|year= 1994|title= Ontogeny, Intraspecific Variation, and Systematics of the Late Cambrian Trilobite Dikelocephalus|journal= Smithsonian Contributions to Paleobiology|volume= 79|pages= 89|accessdate= 2014-03-07|url= http://www.sil.si.edu/smithsoniancontributions/Paleobiology/pdf_hi/SCtP-0079.pdf|doi=10.5479/si.00810266.79.1}}
3. ^[https://books.google.com/books?id=UM4XViDlrf0C&pg=PA358&lpg=PA358&dq=%22progressive+reduction+of+variability%22+%22daniele+rosa%22&source=bl&ots=aH2T4YBOav&sig=7myRW8Rw8A84ylw8yg4E6a8HrDA&hl=en&sa=X&ei=OWNBU5azArjIsASsm4GIAw&redir_esc=y#v=onepage&q=%22progressive%20reduction%20of%20variability%22%20%22daniele%20rosa%22&f=false Life's Splendid Drama: Evolutionary Biology and the Reconstruction of Life's Ancestry, 1860-1940], by Peter J. Bowler; published 1995 by University of Chicago Press (via Google Books)