LAMARCK’S THEORY OF EVOLUTION 243 
of external circumstances, and give rise to a diversity 
of species so considerable and so singularly ordered 
that instead of being able to arrange them, like the 
groups, in a single simple linear series under the form 
of a regular graduated scale, these very species often 
form around the groups of which they are part lateral 
ramifications, the extremities of which offer points 
truly isolated. 
‘There is needed, in order to change each internal 
system of organization, a combination of more influ- 
ential circumstances, and of more prolonged duration 
than to alter and modify the external organs. 
“T have observed, however, that, when circum- 
stances demand, nature passes from one system to 
another without making a leap, provided they are 
allies. It is, indeed, by this faculty that she has 
come to form them all in succession, in proceeding 
from the simple to the more complex. 
“It is so true that she has the power, that she 
passes from one system to the other, not only in two 
different families which are allied, but she also passes 
from one system to the other in the same individual. 
“The systems of organization which admit as organs 
of respiration true lungs are nearer to systems which 
admit gills than those which require trachee. Thus 
not only does nature pass from gills to lungs in allied 
classes and families, as seen in fishes and reptiles, but 
in the latter she passes even during the life of the same 
individual, which successively possesses each system. 
We know that the frog in the tadpole state respires | 
by gills, while in the more perfect state of frog it re- 
spires by lungs. We never see that nature. passes 
from a system with trachee to a system with lungs. 
‘Tt ts not the organs, t.e., the nature and form of 
the parts of the body of an animal, which give rise to 
the special habits and faculties, but, on the contrary, tts 
habits, its mode of life, and the circumstances in which 
wndividuals are placed, which have, with time, brought 
