LAMARCK’S THEORY OF DESCENT 2O7 
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.” 
“Vet, to facilitate the study and knowledge of so 
many different organisms it is useful to give the name 
of species to every similar collection of similar indi- 
viduals which are perpetuated by heredity (géxéra- 
dion) 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 species in the way already 
described in Chapter III. (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 
