VI INTRODUCTION. 



present we possess no such knowledge as this ; but what is 

 known affords sufficient data for a physiological classifica- 

 tion, which, if not entirely satisfactory, nevertheless ap- 

 proximates sufficiently nearly to a satisfactory arrangement 

 to fulfil the objects of classification to a very considerable 

 extent. Although the causes producing malformations are 

 themselves hidden in the deepest obscurity, their effects 

 are sufficiently definite to admit of a very clear diagnosis 

 and arrangement, whenever the first deviation of structure 

 can be perceived and studied apart from the various modifi- 

 cations which succeed and depend upon it. 



The primary deviation from the normal type may be 

 looked upon as the immediate cause of the malformation. 

 All the successive changes and aberrations which depend 

 upon it bear a distinct morphological relation to each other 

 and to the primary deviation. Hence when malformations 

 belong to the same group (that is, when the primary de- 

 viations are the same), they bear a distinct morphological 

 relation to each other and admit of a morphological classi- 

 fication. 



From these considerations it is evident that malforma- 

 tions due to the same cause affecting morphologically similar 

 parts may be arranged in groups, the members of which 

 are morphologically related, although the groups themselves 

 can only be arranged in classes by a knowledge of the 

 physiological relation which they bear to each other. Such 

 an arrangement is possible in many cases, because analogy 

 leads to the belief that the causes determining the occur- 

 rence of the malformations included in these groups are 

 similar. In some the first deviations from the normal 

 type are clearly of the same nature, or due to identical or 

 nearly identical causes ; we are then able to class the re- 

 sulting forms with certainty in one class ; but a number 

 occur in which the primary deviation is too obscure to admit 

 of this. Hence Bischoff argued that no satisfactory classi- 

 fication could be drawn from any but purely anatomical 

 grounds, and many authors have followed this principle 



