422 Prof. E. Hackel on the CalcispongiEe 



characters of form occurring in this group in their general con- 

 nexion, and to explain the perfect "unity of their plan of 

 structure" by the coinmon descent of all Calcispongicefrom the 

 Olynthus. In the second volume, on the other hand, I have 

 sought to demonstrate the stock-relationship of all the forms 

 of this group by subjecting the species of Calcispongise to the 

 most exact anatomical analysis ; in doing which I found my- 

 self compelled, in opposition to the existing rules of classifi- 

 cation, to set side by side two perfectly different systems, a 

 natural and an artificial one. 



The principles of classification which I have followed will 

 manifest themselves to the thoughtful reader from a comparative 

 study of the two systems. The natural system is " carried out 

 in accordance with the phylogenetic principles of the theory of 

 descendence, with an average extension of the idea of species." 

 It contains 21 genera with 111 species. The artificial system 

 is ''carried out in accordance with the principles hitherto 

 followed in the classification of the sponges, with an average 

 extension of the idea of species." It includes 39 genera with 

 289 species. 



The logical principles upon which the artificial system is 

 founded are quite different from the genealogical principles 

 upon which the natural system rests. The former takes into 

 consideration especially the products of adaptatiouy the latter 

 the constancy of inheritance. The artificial system furnishes 

 as definite a distinction as possible, and a summary arrangement 

 of the various forms founded on those characters which strike 

 one as specific characters on a logical comparison merely 

 directed to the external morphological connexion of the forms. 

 The natural system, on the contrary, strives after the more 

 profound recognition of their internal morphological connexion, 

 and seeks, in accordance with this, to approach the genealogical 

 tree of the species. As a matter of course, this object will never 

 be completely attained among the sponges, any more than with 

 other organisms, for the simple reason that the three great 

 documents of the natural history of Creation (Comparative 

 Anatomy, Ontogeny, and Paleontology) are accessible to us 

 only in imperfect fragments. Nevertheless, by continued 

 phylogenetic attempts, the natural system will gradually ap- 

 proach more and more to the true genealogical tree. 



How far this approximation has been successful in the natural 

 system of the Calcispongiie, the thoughtful reader will best see 

 by the study of the second volume, and especially from the 

 estimation of the generic and specific, connective and transitory 

 varieties. The approximation to the true genealogical tree is 

 more possible than with other groups of organisms, because 



