No. 381.} A HALF-CENTURY OF EVOLUTION. 633 
While the doctrine of the effect on animals of a change of 
environment was suggested very early in this century and 
forms the corner stone of Lamarckism, Wallace was, after 
De la Beche! and especially Lyell,? the first in recent times, 
in an essay published in 1855, to call attention to this subject 
thus : 
“To discover,” he says, “how the extinct species have from time to 
time been replaced by new ones down to the very latest geological period, 
is the most difficult, and at the same time the most interesting problem in 
the natural history of the earth.” 3 
Still more recently he remarks: 
“ Whenever the physical or organic conditions change, to however small 
an extent, some corresponding change will be produced in the flora and 
fauna, since, considering the severe struggle for existence and the complex 
relations of the various organisms, it is hardly possible that the change 
should not be beneficial to some species and hurtful to others.” 
Two conclusions are now generally accepted: the first is, 
that the most complete evidence of evolution is afforded by 
paleontology. Huxley’s vigorous affirmation that the primary 
and direct evidence in favor of evolution can be furnished only 
by paleontology has been greatly strengthened by recent dis- 
coveries. The second is, that biological evolution has been 
primarily dependent on physical and geological changes. 
It may not be unprofitable for us as zodlogists to pass in 
review some of the revolutions in geological history, particu- 
larly as regards our own continent, some important details of 
which have recently been worked out by geologists, and to 
note the intimate relation between these revolutions and the 
origination not only of new species but of new faunæ, and 
indeed, at certain epochs, of new types of organic life. 
1. Precambrian Revolutions. — That immensely long period 
which intervened between the time when our planet had cooled 
down and become fitted for the existence of animal life, and 
the opening of the Cambrian period, was evidently a time of 
the geologically rapid production of ordinal and class types of 
1 Researches in Theoretical Geology, p. 217. New York, 1837. Quoted by 
Woodworth, p. 220. 8 Natural Selection, p. 14. 
2 Principles of Geology. 1839. 4 Darwinism, p. 115. 1889. 
