INTRODUCTION ih! 
have become evolved from a single primitive cell, or from a few primitive 
types. 
According to the Linné-Cuvier doctrine, a species is composed of individuals 
which are directly descended from one another, or from common ancestors, 
and which resemble their progenitors as much as they resemble each other. 
Members of one and the same species interbreed, while individuals belonging 
to different species do not cross, or when they do, produce infertile or imper- 
fectly fertile offspring. 
According to the theory of descent, no sharp specific distinctions can be 
drawn, but all individuals are assigned to the same species which possess a 
number of essential properties in common, and which are not connected on 
all sides with neighbouring groups by means of intermediate types. It is 
plain that this definition is open to considerable laxity of interpretation, and 
inasmuch as the direct descent of individuals belonging to a given species 
cannot always (in palaeontology never) be determined on experimental grounds, 
systematists are rarely fully agreed in regard to the limitations of species, 
genera, and families. 
The doctrine of the invariability of species received powerful support from 
the cataclysmic theory of Cuvier, which maintained that each period in the 
earth’s history is marked by distinctively characteristic faunas and floras ; that 
no species is common to two successive periods; that tremendous convulsions 
of nature (cataclysms) occurred at the close of each cycle, and annihilated the 
whole organic world ; and that by means of special creative acts, the renovated 
earth became time and again populated with new animals and plants which 
bore absolutely no connection either with previous or with subsequently 
introduced types. 
Cuvier’s cataclysmic theory may be regarded at the present day as com- 
pletely overthrown, inasmuch as the modern school of geology, following the 
leadership of Sir Charles Lyell, has demonstrated conclusively that the earth 
has proceeded from one stage to another during the course of its development 
only with the utmost slowness; that the same forces and laws which regulate 
the world of to-day have operated likewise in primeval times; and that geo- 
logical periods are by no means abruptly set off from one another, but, on the 
contrary, are linked together by innumerable transitional stages. 
The theory of the descendant origin of organic forms, which was advanced 
as early as 1802 by J. B. Lamarck and Geoffroy St. Hilaire, and was supported 
by Géthe, Oken, and Meckel in Germany, kept winning continually more and 
more adherents, yet it was not until the latter half of the present century that 
its universal significance was insisted on by Charles Darwin and his school. 
Palaeontology, as already remarked, contributes a great deal of extremely 
weighty evidence in favour of the theory of descent ; the series of intermediate 
forms, often traceable through several successive formations ; the presence of 
embryonic and generalised types; the parallelism between ontogeny and the 
chronological succession of related fossil forms; the similarity between floras 
and faunas of approximately the same age; the correspondence in the geo- 
graphical distribution of recent organisms with that of their progenitors ; and 
a host of other facts are explicable only by means of the theory of descent. 
The causes of variation and transmutation were attributed by Lamarck 
chiefly to the use and disuse of organs ; secondly, to the effect of changes in ex- 
ternal conditions ; and lastly, to a supposed resident tendency toward variation 
