390 LAMARCK'S THEOEY 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 pre-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 

 deviating, in the course of ages, so far from their original 

 type 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 colour, form, structure, physiological attri 

 butes, and even instincts, were gradually modified by expo 

 sure to new soils and climates, new enemies, modes of 

 subsistence, arid 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 naturally, as 

 when a species, on the extreme verge of its geographical 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 taught not only that species had been con 

 stantly undergoing changes from one geological period to 

 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 complex struc- 



