INTRODUCTION 15 



other requisites for its support ; and these conditions govern its distribution 

 ami 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 



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 Belated 

 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.] 



