INTRODUCTION 15 
other requisites for its support; and these conditions govern its distribution 
and increase in the last degree. Every plant controls the destiny of all 
animals subsisting upon it; their numbers multiply with its increase, and 
wane with its decrease. The fate of these creatures determines that of their 
natural enemies, who stand in similar relationships to still remoter circles ; 
and hence no form can overstride the bounds set for it by the general balance 
without disturbing the whole general system of economy. Let the flora or fauna 
of a given region become altered by the extinction of a number of species, or 
by the introduction of new and more powerful competitors, the balance is 
immediately upset. In the first instance vacant places must be filled up, and 
in the second, room must be made for the newcomers at the expense of the 
settled community. Thus, wherever climatal, orographic, or other changes 
are instrumental in bringing about the extermination of large numbers of 
plants and animals during the lapse of a geological period, an inequilibrium 
must necessarily result. But thereupon the struggle for existence is waged 
with unwonted severity among the survivors, until finally a new state of 
equilibrium is attained, and a pause in the formation of new species ensues. 
The whole course of evolution in the organic world during past geological 
periods indicates not only definite progression in all branches of the animal 
and vegetable kingdoms up to their present state, but also an advance toward 
perfection. Granting that the theory of descent is true, and that all organisms 
have developed from a single primitive cell, or from a few primitive ground- 
types, then every new growth and differentiation must stand for improvement 
and progress, leading gradually to the development of more or less highly 
specialised organs, and to a division of labour in their physiological functions ; 
the higher the degree in which this is manifested, and the more conformably 
to apparent purpose and utility that each organ fulfils its functions, the more 
perfect is the organism, as we conventionally term it. Evolution in the 
organic world has not advanced in a simple, straightforward direction, but in 
an exceedingly complicated and circuitous. The biological systems, accord- 
ingly, do not suggest to us the similitude of a ladder with its numerous 
rounds, but rather that of an enormously ramifying tree, whose topmost 
twigs represent the youngest, and, on the whole, the most perfect forms of 
every branch. The root, trunk, and a goodly portion of the upper limbs lie 
buried in the earth; and only the ultimate green shoots, the last and most 
highly differentiated members of long ancestral lines, blossom forth in the 
world of to-day.? 
1 [In connection with the two preceding topics, see a paper by the author, read before the Inter- 
national Congress of Geologists, 1894, on “ Palaeontology and the Biogenetic Law” (reprinted in 
Natural Science, vol. VI., May 1895). 
On the terminology of evolution in general, see A. Hyatt, “Bioplastology and the Related 
Branches of Biologic Research” (Proc. Bost. Soc. Nat. Hist. vol. XXVI., 1893). Abstract of same 
in Zoolog. Anzeiger, No. 405, 1892. Other terms employed in the foregoing are introduced and 
explained by Cope in his “Origin of the Fittest,’ 1887, and in various articles in the American 
Naturalist.—TRans. ] 
