o90 Lamarck's theory chap. xx. 



of descent. He therefore proposed that the element of time 

 should enter into the definition of a species, and that it 

 should run thus: — "A species consists of individuals all re- 

 sembling each other, and reproducing their like by genera- 

 tion, so long as the surrounding conditions do not undergo 

 changes sufficient to cause their habits, characters, and forms 

 to vary." He came at last to the conclusion that none of the 

 animals and plants now existing were primordial creations, 

 but were all derived from pi'e-existing forms, which, after 

 they may have gone on for indefinite ages reproducing their 

 like, had, at length, by the influence of alterations in climate 

 and in the animate world, been made to vary gradually, and 

 adapt themselves to new circumstances, some of them de- 

 viating, in the course of ages, so for from their original tj'jie 

 as to have claims to be regarded as new species. 



In support of these views, he referred to wild and culti- 

 vated plants, and to wild and domesticated animals, pointing- 

 out how their color, form, structure, physiological attributes, 

 and even instincts, were gradually modified by exposure to 

 new soils and climates, new enemies, modes of subsistence, 

 and kinds of food. 



Nor did he omit to notice that the newly acquired peculi- 

 arities may be inherited by the offspring for an indefinite 

 series of generations, whether they be brought about natu- 

 rally, — as when a sj^ecies, on the extreme verge of its geo- 

 graphical range, comes into competition with new antagonists 

 and is subjected to new physical conditions; or artificially, — 

 as when, by the act of the breeder or horticulturist, peculiar 

 varieties of form or disposition are selected. 



But Lamarck tauglit not only that species had been con- 

 stantly undergoing changes from one geological period t(» 

 another, but that there also had been a progressive advance 

 of the organic world from the earliest to the latest times, 

 from beings of the simplest to those of more and more com- 



