16 ELEMENTS OF PALEONTOLOGY 



entirely disappears, or is replaced by closely related, but at the same time 

 more or less different forms. Obviously, therefore, there have been periods 

 when the process of transformation and the vi^eeding out of organisms were 

 greatly accelerated, and following upon these reconstructive periods long 

 intervals of repose have ensued, during which intervals species have retained 

 their characteristic forms with but little variation. The fact that evolution 

 has advanced by occasional bounds or leaps stands, however, in nowise contra- 

 dictory to the theory of descent. 



T^e whole animate community at any point on the earth's surface rests 

 normally in a state of equilibrium, the balance being maintained by the com- 

 bined activity of all ranks and members of society. For the preservation of 

 this balance nature practises a most rigid domestic economy. Every plant 

 depends upon particular conditions of soil, food, temperature, moisture and 

 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, a state of inequi- 

 librium must necessarily result. But thereupon the struggle for existence 

 is waged with unwonted severity among the survivors, until finally a 

 readjustment is established, 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 a more perfected 

 specialisation. 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 im- 

 provement 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 its course has been exceedingly complicated and circuitous. 

 The biological systems, accordingly, do not suggest to us the similitude of a 

 ladder with its numerous rounds, hwt 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. 



