152 BIOLOGICAL LECTURES. 



practice authors are apt to blunder and insert wrong numbers 

 in the text. One set of references has to be used while 

 writing, and when all is done another set substituted, and 

 during the substitution mistakes are not infrequent. The 

 other system was introduced by Professor E. L. Mark, and has 

 met with increasing favor. In this system each article is 

 identified by the name of the author, the date of publication, 

 and an arbitrary catalogue sign, which last may be a single 

 letter or digit. For example : 



MARK, E. L. 81.1. Maturation, Fecundation, and Segmentation of 

 Limax campestris, Binney. Bull. Mus. Comp. Zool., VI, 171-625, 

 Pis. I-V. 



indicates all that is necessary. The figures 81.1 stand for the 

 year 1881, the digits indicating the century being omitted for 

 all years of the present century ; and the single digit after the 

 period indicates the arbitrary order of entry, in this case 

 that the article in question is the first one to be recorded for 

 that author and year. Should a second article published by 

 Professor Mark in 1881 appear in the list, it would be 81.2; but 

 were it in 1882, it would be 82.1. Professor Mark uses a 

 slightly different notation, letters serving for entry signs ; thus 

 the paper just cited would be recorded 



MARK, E. L., '8i a , 



or, 



8i a . MARK, E. L. 



The apostrophe indicates that the two digits are omitted, but 

 now that this system has been widely used, the apostrophe may 

 be safely left off. If one uses Mark's system, the references 

 may be all put in as the manuscript is written, and the articles 

 will then group themselves. Every worker should have a card- 

 catalogue of the publications in his own field, and to such a 

 catalogue Mark's system is peculiarly adapted; and if it is used, 

 it is easy to enter all references in one's manuscript, giving 

 each paper its notation from the catalogue. While preparing 

 my Human Embryology I utilized this plan for the several 

 thousand references made in the course of the work, and can 



