GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 407 
The deep arm of ocean between Florida and Cuba or the Bahamas has 
proved ineffectual in checking their dispersions. What is the more 
potent barrier that prevents their northward spread along the continu- 
ous land of the peninsula? The answer is summed up in the single 
word climate. The temperature of the period of growth and reprodue- 
tion in the northern parts of Cuba and the Bahamas is the same as in 
sub-tropical Florida, but to the northward it falls off rapidly. 
Respecting Wallace’s statement that the difference between the 
faunas and floras of hot and cold countries “is by no means constant,” 
and does not bear ‘“‘any proportion to difference of temperature,” it 
need only be said that no phenomenon of nature is more constant; and 
that the differences observed depend directly upon temperature. Presi- 
dent D. S. Jordan has said: “In many groups, anatomical characters 
are not more profound or of longer standing than are the adaptations 
toheatand cold.” (Popular Science Monthly, Aug., 1890, Xxxvil, p. 506.) 
That ‘life is distributed in circum-polar zones, which conform with 
the climatie zones, though not always with the parallels of the geog- 
rapher” is a law recognized by Humboldt, Wagner, Agassiz, Dana, 
De Candolle, Allen, and nearly all writers on distribution except 
Wallace. This law does not imply that the same species, genera, or 
higher groups recur under the same degree of heat in disconnected 
land areas—a manifest impossibility,—but that well-marked zones of 
animal and plant life are encountered in all parts of the earth in pass- 
ing from the poles to the tropics; that they owe their existence to con- 
stant differences of temperature, and that in continuous land areas 
each zone may be traced completely across such areas (from ocean to 
ocean in those of continental magnitude), following the windings of the 
belts of equal temperature during the period of reproductive activity. 
Wallace speaks thus of this law as formulated by Allen: “The 
author (J. A. Allen) continually refers to the ‘law of the distribution 
of life in circum-polar zones,’ as if it were one generally accepted and 
that admits of no dispute. But this supposed ‘law’ only applies to the 
smallest detail of distribution—to the range and increasing or decreas- 
ing numbers of species as we pass from north to south, or the reverse; 
while it has little bearing on the great features of zodlogical geogra- 
phy—the limitations of groups of genera and families to certain areas.” 
(Geog. Dist. of Animals, 1876, vol. 1, p. 67.) Mr. Allen has already 
pointed out the weakness of this criticism (Bull. U. S. Geol. and Geog. 
Survey Terr., May, 1878, vol. tv, No. 2, 326), and I would like to add a 
word respecting the extraordinary statement that cireum-polar distri- 
bution affects species only, having “little bearing” on the “ limitations 
of groups of genera and families.” In refutation of this fallacy it is 
hardly necessary to do more than call attention to the circumstance 
that the trans-continental Sonoran region of North America is distin- 
guished from the Boreal by the possession of seven families and thirty- 
