LAMARCK'S THEORY OF DESCENT 287 



places which suit them are less subject than many 

 other animals to the variations of local circumstances, 

 and hence less restricted in their habits." 



He adds the fact that the animals in question have 

 inhabited Egypt for two or three thousand years, and 

 not necessarily from all time, and that this is not 

 time enough for marked changes. He then gives 

 the following definition of species, which is the best 

 ever offered : " Species, then, have only a relative 

 stability, and are invariable only temporarily." 



" Yet, to facilitate the study and knowledge of so 

 many different organisms it is useful to give the name 

 of species Xo every similar collection of similar indi- 

 viduals which are perpetuated by heredity {g^n^ra- 

 tioii) in the same condition, so long as the circum- 

 stances of their situation do not change enough to 

 render variable their habits, character, and form." 



He then discusses fossil specie:: in the way already 

 described in Chapter HI. (p. 75). 



The subject of the checks upon over-population 

 by the smaller and weaker animals, or the struggle 

 for existence, is thus discussed in Chapter IV. : 



" Owing to the extreme multiplication of the small 

 species, and especially of the most imperfect animals, 

 the multiplicity of individuals might be prejudicial to 

 the preservation of the species, to that of the progress 

 acquired in the improvement of the organization — in 

 a word, to the general order, if nature had not taken 

 precautions to keep this multiplication within due 

 limits over which she would never pass. 



" Animals devour one another, except those which 

 live only on plants; but the latter are exposed to 

 being devoured by the carnivorous animals. 



" We know that it is the strongest and the best 



