IV PREFACE. 



Two dangers have always appeared to me equally 

 formidable in the study of the structure of organized 

 beings : the one is to comprehend it a priori in a 

 manner too abstract and too general, and to make it 

 subordinate, either to analogies too distinct, or to meta- 

 physical ideas too uncertain : it is with this that several 

 of those may be upbraided who disdain the study of 

 facts in order to beheve the philosophy of nature. The 

 other danger is that of only looking upon the structure 

 of beings as isolated facts, and not endeavouring to con- 

 nect them by any theory : it is with this that the simply 

 descriptive school may be reproached. 



The true way, it seems to me, lies between these two 

 extremes ; there must be here, as in all other sciences, 

 particular facts, regulated by laws at first partial, which 

 gradually become more general, and will one day per- 

 haps be universal. We may thus ascend, by the succes- 

 sive arrangement of facts, even to theories, some of 

 which may have been conceived by philosophers, but had 

 not as yet been supported by sufficient proofs ; exactly 

 as from the knowledge of the laws of organization, we 

 may descend to the examination of facts which had 

 been seen by observers, but the connexions of which 

 were not understood. I even doubt that any one can 

 form any correct theories, if he have not been brought 

 up habitually to the study of facts ; or that he can 

 make completely useful descriptions, if he have neglected 

 entirely the theories which those descriptions ought to 

 elucidate. 



When we compare, in this point of view, the two 

 great schools I have just mentioned, we see with sur- 

 prise that the first is devoted to the study of the con- 

 nexions of the structure of organs, and that it almost 

 entirely neglects the relations of comparison derived 

 from the collection of beings ; while the second, entirely 



