ARRANGEMENT AND CLASSIFICATION 59 



These irregularities in the perfection and degradation of inessential 

 organs are found in those organs which are the most exposed to the 

 influence of the environment ; this influence involves similar irregu- 

 larities in the shape and condition of the external parts, and gives 

 rise to so great and singular a diversity of species that, instead of being 

 arranged like the main groups in a single linear series as a regularly 

 graduated scale, these species often constitute lateral ramifications 

 around the groups to which they belong, and their extremities are 

 in reaUty isolated points. 



A much more powerful and lasting set of conditions is necessary to 

 modify any internal system of organisation than to alter the external 

 organs. 



I observe, however, that in cases of necessity nature passes from one 

 system to another without a break, if they are closely alhed ; it is 

 indeed by this faculty that she succeeded in fashioning them all in 

 turn, passing from the simplest to the most complex. 



So true is it that she has this faculty, that she even passes from one 

 system to another not merely in two different alhed famiUes but in 

 one individual. 



Those systems of organisation in which respiration is carried on by 

 true lungs are nearer to the systems requiring gills than to those requir- 

 ing tracheae ; thus, nature not only passes from gills to lungs in alUed 

 classes and famihes, as is seen among fishes and reptiles, but she does 

 so even during the existence of one individual : which possesses in 

 turn first one and then the other system. It is known that the frog, 

 in its imperfect condition of tadpole, breathes by gills ; while in its 

 more perfect condition of frog it breathes by lungs. But nowhere 

 does nature pass from the system of tracheae to the pulmonary 

 system. 



It may then be truly said that in each kingdom of living bodies the 

 groups are arranged in a single graduated series, in conformity with the 

 increasing complexity of organisation and the affinities of the object. 

 This series in the animal and vegetable kingdoms should contain the 

 simplest and least organised of hving bodies at its anterior extremity, 

 and ends with those whose organisation and faculties are most perfect. 



Such appears to be the true order of nature, and such indeed is 

 the order clearly disclosed to us by the most careful observation and 

 an extended study of all her modes of procedure. 



We have seen the necessity of paying attention to the question of 

 affinities, in drawing up our arrangements of the productions of nature ; 

 hence we are no longer able to arrange the general series in any way 

 we hke. Our knowledge of nature's methods continues to increase 

 in proportion to our studies of the affinities between objects or various 



