LAMARCK'S THEORY OF JE VOLUTION 



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. 



" I 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 

 pdjrit gills than those which require tracheae. Thus 

 not only does nature pass from gills to lungs in allied 

 classes and famiUes, 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__ f rog io _ the _tadpole state respires 

 by^lls, while in the more perfect state of frog_ it re- 

 spires by lungs. We never see that nature passes 

 from a system with tracheae to a system with lungs. 



"It is not the organs, i.e., the nature and form oj 

 the parts of the body of an animal, which give rise to 

 the special habits and faculties, but, on the contrary, its 

 habits, its mode of life, and the circumstances in which 

 individuals are^f>laced, which have, with time, brought 



