xxviii Field Columbian Museum — Zoology, Vol. V. 



Thus, in a measure at least, species of fishes in Mexico are under- 

 going changes, and the process of making species or varieties is still 

 actively in progress. There is in general more individual variation 

 in species of Mexican fresh-water fishes than among the species which 

 inhabit the Mississippi Valley. To define properly the species and 

 subspecies of fishes in the country in question a much more extended 

 study of each is necessary, and for this reason I have not recognized 

 any subspecies in the present paper. 



A few species, as Hybognathus episcopus (Girard), have a wide dis- 

 tribution, besides this they range from near sea level to an altitude of 

 6,000 or 7,000 feet. If in this case we recognize any subspecies at 

 all, we must recognize one for each important stream in which the 

 species occurs. In such cases I have recognized one species and noted 

 the important, though slight, differences in the specimens from the vari- 

 ous localities from which I have material. For examples of extreme 

 individual variation of color markings, I would refer to Platy- 

 pcecilus maculatus Gunther, and Platypcecilus variatus Meek. The 

 student of Mexican fresh-water fishes must be constantly on the out- 

 look for these variations. In defining species it is certainly better to 

 rely on structural differences rather than on differences in color. On 

 many of the tropical fishes, especially in the streams of the lower 

 lands, there are black blotches which appear more like stains than 

 markings. These are not at all definite in outline, or in position; 

 some may be large, others small, and their position without regularity. 

 An example of this kind is seen in Xiphophorus helleri Gunther, of 

 which a variety noted by Dr. A. Gunther, because of these black blotches, 

 was later for the same reason regarded as a distinct species* by Jordan 

 & Evermann. Many of the Pceciliidce have these blotches, although 

 they are not uncommon in other species. Unless an apparently new 

 species is very different from its nearest relative, it is quite as well 

 not to describe it as new unless a large amount of material is at hand 

 for comparison. I am not certain that I have properly followed this 

 rule myself, still a careful examination of my material will, I believe, 

 show that I have not materially departed from it. 



LIST OF LOCALITIES IN MEXICO WHERE COLLECTIONS 

 OF FISHES WERE MADE IN 1901 BY MR. F. E. LUTZ 

 AND THE AUTHOR. 



Colonia Juarez, Chihuahua; Rio Casas Grandes, June 26. 

 Guzman, Chihuahua; spring near railroad station, June 25. 

 Santa Maria, Chihuahua; springs and ponds near clubhouse, June 24. 

 San Jose, Chihuahua; spring at San Jose ranch, June 21. 



*Xiphophorus guntheri Jordan & Evermann. 



