INTRODUCTION 11 



and which resemble their progenitors as much as they resemble each other. 

 Members of one and the same species interbreed, b\it individuals belonging 

 to diiferent species do not cross, or when they do, produce infertile or 

 imperfectly 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 share a 

 number of essential features in common, and which are not connected 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 

 paleontology never) be determined on experimental grounds, systematists 

 are rarely agreed in regard to the precise 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 with either previous or subsequently introduced types. 



Cuvier's cataclysmic theory may be regarded at the present day as com- 

 pletely ovei'thrown, 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 

 geological periods are by no means abruptly set off from one another, but are 

 linked together by innumerable transitional stages. 



The theory of the descendant origin of organic forms, Avhich was advanced 

 as early as 1802 by J. B. Lamarck and Geoftroy St. Hilaire, and was supported 

 by Goethe, Oken and Meckel in Germany, kept winning continually more 

 adherents, yet it was not until the latter half of the nineteenth century that 

 its universal significance was insisted on by Charles Darwin and his school. 



Paleontology, 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 

 primitive 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 

 geographical 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 transformation were attributed by Lamarck 

 chiefly to the use and disuse of organs ; secondly, to the effect of changes in 

 external conditions ; and lastly, to a supposed inherent tendency toward 

 valuation and perfection existing in each individual. According to Lamarck, 

 new characters brought about by these influences are transmitted to descend- 

 ants through inheritance, and become permanently established in the I'ace. 

 Geoffroy St. Hilaire maintained the same principles on the whole, but ascribed 

 the chief causes of variation of species to the influence of environment. 



