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INTRODUCTION. 



adaptational forms, or according to the systematic divisions. Whether this attempt 

 can ever succeed, will depend upon the results of further investigations, which shall 

 have extended over whole families and classes, having regard more comprehensively 

 and completely to all the questions concerned, than has hitherto, as a rule, been the 

 case. According to the present state of our knowledge there remains for the state- 

 ment of the anatomical peculiarities of the groups, which may be distinguished 

 according to natural relationship or direct adaptation, only this one tolerably con- 

 sistent and practicable course, to start from the tissues and their arrangement, and 

 to introduce into the general consideration of these the rules and exceptions which 

 obtain for the two kinds of groups above named. 



