REVIEWS By 
through the action of natural selection, but this adaptation is not the 
evolution nor is natural selection the cause of evolution, but only one 
of the factors in the process of evolution. A first cause of some sort 
is essential to any complete theory of evolution. 
“Among those today who adopt evolution as the explanation of the 
mode of origin of the different forms of organisms, there are two 
extremes of opinion with many intermediate compromises. 
“All will agree in recognizing ancestry and environment as each 
taking some part in the evolution; but the extreme school, on the one 
hand, holds that exvéronment is the chief factor determining the 
direction and extent of the modifications, which heredity tends to per- 
petuate, and that ancestry plays only the part of holding and preserv- 
ing, in its offspring, what it gets from the agency of environment. 
‘The other extreme is the opinion that avcestry is the more efficient 
factor in bringing about the evolution ; that in what is called varzability 
there is working out, not a mere accidental reflex of environment upon 
the plastic organism, but a fundamental property or force of organisms, 
ever tending from homogeneity to heterogeneity and resulting in the 
specialization of functions and the differentiation of organic structure 
always; the line of evolution followed out by any particular race being 
influenced little by environment,—the adjustments being active and 
not passive, — the successful organisms seeking and adopting conditions 
favorable for their existence if out of them, dying out if the conditions 
favorable are not within reach, or if crowded out of them. Natural 
selection to this school of opinion plays rather an eliminating rdle 
than one of causation, and explains rather why there are gaps in the 
series of organisms than why the characters assumed in the modified 
forms are what they are. In this latter view the successive steps of 
modification of a race are as much controlled by the ancestry as are the 
successive steps of development inthe growth of the individual. 
“In the former view there is the replacement of the theory of 
immutability of species by that of the mutability of species, but the 
process of reproduction is still looked upon as immutable, reproducing 
the characters of the parents in the offspring without change; in the 
second view reproduction itself takes a part in evolution and normally 
accomplishes modification of form, either slowly or suddenly, but 
progressively, and evolution is an intrinsic law of organism.” 
Mutability of species is the central thought in the new theory of 
the origin of species. Darwin first clearly announced that specles are 
