TA INTRODUCTION. 
Naturalists commonly complain of dearth of material: I 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 been made up into about 
thirty sets for distribution by the Institution. A few 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 
uaturalist 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 it. 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 ciaim 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 be 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 
