108 
on the basis of something more than a 
superficial knowledge of its content, can 
do so with all sincerity. It is legitimate 
that we should struggle to make it as popu- 
lar a science as may be without discarding 
its essentially rigorous methods, for, as Dr. 
Nutting has said, the typical product of 
slack methods is a slacker.** But difficul- 
ties will only serve to heighten its esti- 
mated value, once it becomes generally 
known that physics is good for something. 
In meeting the demand for such evidence, 
the physics teacher will find the greatest 
opportunity for his own development and 
that of his beloved science. 
E. H. JoHNSON 
THE IRWIN EXPEDITION OF INDIANA 
UNIVERSITY TO PERU AND BOLIVIA 
In 1909 I summarized the knowledge of the 
distribution of South American fresh-water 
fishes in general. I dealt with the origin of 
the Pacific slope fish fauna in part in the fol- 
lowing words :? 
There are four distinct faunas on the Pacific 
slope of America between Cape Horn and the 
Tropic of Cancer. One of these is of common 
origin with that on the Atlantic slope, one is au- 
tochthonus and the other two are derivative from 
the Atlantic slope faunas opposed to them. 
1, The fauna of southern Chili is essentially like 
that of Patagonia, and inasmuch as it is largely 
made up of marine forms entering fresh water, 
and fresh-water forms entering the ocean, it seems 
very probable that the species migrated from river 
to river along the coast from Patagonia to Chili or 
from Chili to Patagonia. i 
2. At the other extreme in the Rio Mezquital of 
the Transition Region and the Yaqui just to the 
north of it there is a fauna essentially like that 
of the Rio Grande east of them. As Meek has 
pointed out, the Yaqui and Mezquital have cap- 
tured tributaries of the Rio Grande togther with 
the fishes in them, and the migration of Atlantic 
slope northern forms to the Pacific slope has been 
a passive one, 
34 Scientific Monthly, May, 1918, p. 406. 
1 Reports of the Princeton University Expedi- 
tions to Patagonia, IIT., 1909, p. 352. 
SCIENCE 
[N. S. Vou. XLVIII. No. 1231 
Thus, types which in America north of Mexico 
have not succeeded in reaching the Pacifie slope, 
have, within the Tropics, crossed the divide. . . . 
3. The third fauna is the Mexican of the Rio de 
Santiago. This is undoubtedly the relict of an 
old fauna reenforced by a few immigrants from 
the north. It is here not a question of the origin 
of the fauna from an eastern one, but of an au- 
tochthonus development that has, on its part, con- 
tributed elements to the surrounding rivers. It 
passively contributed to the Atlantie slope fauna 
by having one of its small rivers captured by the 
Rio Panuco. 
4. Of more particular interest is the origin of 
the fauna of western Peru and Heuador and that 
of western Central America. Not enough is 
known of the fauna of the western part of Central 
America to attempt an explanation of its origin. 
Concerning the Andean fauna I said in part, 
page 305: 
The Andean region includes the high Andes on 
both slopes from Venezuela and Colombia to Chili. 
It is poor in species at any given point, but 
some,of the genera have a large number of local 
adaptations or species. This region is distinctly 
marked off into three provinces. 
1. The Northern includes the highlands of north- 
ern Peru, Ecuador, Colombia and Venezuela. This 
is the richest in species and distinguished by the 
genera Arges, Cyclopium, Prenadilla and the high 
development of Chetostomus. Its fauna is largely 
an ancient derivative from the lowland fresh- 
water fauna of Archiguiana. 
2. The Titicacan, including the basin of Titicaca 
and neighboring streams, and possibly the land- 
locked basins of Bolivia, concerning which nothing 
is known, is distinguished by the genus Orestias 
and the absence of the genera distinguishing the 
northern province. Its fauna is largely an ancient 
derivative from the ocean. 
3. The Southern is the poorest in species, char- 
acterized by the absence of everything but a few 
species of Pygidiwm, a genus which extends the 
entire length of the Andean region. 
Further, p. 373, I said: 
The points of strategic importance for ichthyic 
chorology in South America are, therefore, west- 
ern Colombia and Panama, Guayaquil and Peru to 
the Amazon, across the Andes.... 
Most of my time since the publication of 
the monograph quoted, in fact, since its 
preparation several years earlier, has been de- 
