OTHER THEORIES OF SPECIES-FORMING 291 
effects, but remaining uniform in most regards, all representatives 
slowly changing together in the process of adaptation by natural 
selection. In regions broken by barriers which isolate groups of indi- 
viduals we find a great number of related species, though in most cases 
the same region contains a smaller number of genera or families. In 
other words, the new species will be formed conditioned on isolation, 
though these same barriers may shut out altogether forms of life which 
would invade the open district. 
“Given any species in any region, the nearest related species is not 
likely to be found in the same region nor in a remote region, but in a 
neighboring district separated from the first by a barrier of some sort. 
“Doubtless wide fluctuations or mutations in every species are 
more common than we suppose. With free access to the mass of 
the species, these are lost through interbreeding. Isolate them as in 
a garden or an enclosure or on an island, and these may be con- 
tinued and intensified to form new species or races. Any horticul- 
turist will illustrate this. 
“Tn all these and in similar cases we may confidently affirm: The 
adaptive characters a species may present are due to natural selection 
or are developed in connection with the demands of competition. 
The characters, non-adaptive, which chiefly distinguish species do not 
result from natural selection, but from some form of geographical 
isolation and the segregation of individuals resulting from it.” 
J. T. Gulick, another exponent of the efficacy of geographic isola- 
tion in species-forming, has offered in evidence of his views facts about 
the distribution of Hawaiian land snails. In the island of Oahu, for 
example, the volcanic ridges have been eroded out into a series of 
isolated valleys in the bottoms of which grows abundant vegetation, 
while on the highlands there is little but barren rock. The climatic 
conditions of all the numerous valleys are the same, but, remarkably 
enough, each variety of snail is confined not only to one island, but to 
a definite valley on an island. The degree of difference, moreover, 
between varieties is in proportion to the distance that separates them. 
Gulick claimed that he was able to estimate the degree of divergence 
between the snails of any two valleys by measuring the number of 
miles that lay between them. Gulick’s findings have been extensively 
corroborated by recent explorations on the snails of other oceanic 
islands by Crampton. 
An interesting type of isolation that hardly can be termed geo- 
graphic, yet is essentially equivalent to the latter in its effects, is found 
