IV INTRODUCTION. 



Naturalists commonly complain of dearth of material: 1 have been embar- 

 rassed by the enormous amount I have been obliged to examine in order to 

 faithfully execute my self-imposed task. Every specimen has been made to 

 contribute to the general result. The collection has been catalogued, and 

 labeled according to my views; the duplicates have beeu made up into about 

 thirty sets for distribution by the Institution. A tew of the leading sets are 

 only less complete than the Smithsonian reserve series itself; the value of the 

 others successively decreases with lack of the rarer duplicates. 



The results of the investigation being fully — perhaps not without pro- 

 lixity and some repetition — given in the body of the memoir, need not to be 

 here noticed ; I only allude to their entirely original character. I wish, 

 however, to have one word upon the method of study I pursued in this case, 

 as determining the shape which the article finally assumed. The paper 

 is presented very nearly as it was originally prepared, my official engagements 

 having prevented any leisurely revision of the manuscript; and it was written 

 in such odd hours as I could find in the midst of active professional duties. 

 I began the investigation with no more knowledge of* the subject than any 

 naturalist might have incidentally acquired. I had no ''views" to advance, 

 and was entirely free from prejudice. I studied, as I conceive a naturalist 

 should in such cases, with a specimen in one hand and my pen in the other. 

 In taking up the species successively, I never knew, and certainly never 

 cared, what the result would be, being perfectly satisfied to let the specimens 

 tell their own story in their own way. I studied these mice, at intervals, for 

 about a year, and then put my notes together. So the work grew; and if the 

 results be found to square with late progressive views respecting so-called 

 "specific" distinctions, it will be remembered that I am but the mouthpiece 

 of the animals themselves, and claim only the credit of making an accurate 

 report. 



On some accounts, I wish that opportunity had offered to revise and 

 condense an article which will doubtless be more respected for the labor it 

 represents and for its possible value as a contribution to knowledge than 

 as a model of literary handicraft. Yet an honest showing of processes may 

 have its value, as well as a formal exhibition of results. The knowing how 

 a piece of work is done may lie of use in testing its quality. 



The illustrations which accompany this paper are from photographs of 

 the objects directly on sensitized wood, by Smillie's process; the engraving was 



