76 Dr Treviranus on the 



been something more than a mere crude digest of unconnected 

 facts and observations, philosophers have constantly endeavour- 

 ed after the discovery of an arrangement in which the objects 

 of their inquiries should be linked after their natural affinities; 

 while, at the same time, the characters of its subdivisions should 

 possess the utmost simplicity, by being derived from a single 

 system of organs. It may, in a manner, be compared with the 

 philosopher"'s stone, whether he merely resort to such an ar- 

 rangement for the purpose of discovering the name of an un- 

 known animal or plant, or whether, with higher views, his re- 

 searches enter deep into the philosophy of nature. With the 

 first object only in view, external characters alone may suffice, and 

 the more easy these are of detection, the better are they fitted 

 for their purpose. The bond of natural affinities is quite sub- 

 ordinate to this primary object. But if his object be of the 

 other description, the characters may lie deep, and be of the ut- 

 most difficulty to discover. His arrangement will be the more 

 perfect the more completely it expresses the sum of all the ex- 

 ternal and internal structural differences, and the more uniform 

 the parts are from which the characters are taken. 



Of late years systems have been constructed upon the last 

 of these principles. But their bases have been always such 

 as I cannot admit. One of these is the principle. That all 

 higher formations include those lower in the scale ; the most 

 perfect organs in the former having already existed in the latter, 

 but in their undeveloped state ; that there is one universal type, 

 only modified in the degree of its development. There is much, 

 both of truth and of error, in all this. It is true that every or- 

 ganized being advances from the simpler to the more compound 

 in its progressive growth, and that the early stages of the life of 

 an animal high in the scale present many points of similarity with 

 the perfect state of another lower in the series. But these re- 

 semblances hold merely in external relations, and the numerous 

 points of difference can by no means be overlooked. 



The foetus of the mammalia and of birds is an aquatic animal, 

 which respires not by means of lungs, and as such has a simple 

 circulation like a fish. But in all other respects its circulating 

 system is constructed upon a totally different principle from a 

 fish. The apparent analogy vanishes upon a closer inspection. 



