and the Theory of Descendence. 425 



varieties, are of the greatest importance to the theory of de- 

 scendence and the object of this monograph, namely to ascertain 

 analytically the origin of species as exemplified by a single 

 group. The thoughtful and unprejudiced systematist, who 

 has followed carefully the method of classification followed by 

 me in the second volume, will comprehend without further 

 explanation the extraordinary phylogenetic importance of these 

 four different varieties. I may, however briefly sum up the 

 most important points connected with them. 



1. T\\Q generic varieties of the natural system are the genera 

 of the artificial system. Within one and the same natural 

 species many different forms may be developed by multifarious 

 stock-formation and mouth-formation ; and these the artificial 

 system (having no knowledge of their close genealogical con- 

 nexion) must regard unconditionally as representatives not 

 only of distinct species but even of distinct genera. Thus, 

 for example, Ascandra variabilis includes forms Avhich the 

 artificial system would divide among eleven different genera ; 



H. VIII. Artificial system -wdth a very wide extension of the idea of 

 species (in the second degree) : 2 genera with 19 species, namely : — 



I. MoNOGRANTiA, with (j Solitary species : 1 . M, olt/nthus ; 2. 31. disycus; 

 3. M. sycurus ; 4. M. clistolynthus ; 5. M. lipostomella ; 6. M. sycocystis : 



II. PoLYGRANTiA, with 13 social species, namely: 1. P. soleniscus ; 2. P. 

 amphortscus ; 3. P. sycothamnus ; 4. P. nardostis; 5. P. camostomus ; 6. P. 

 tames ; 7. P. artynas ; 8. P. aidopleyma ; 9. P. aphroceras ; 10. P. syco- 

 phyllum; 11. P. asconietra ; 12. P. leitvometra ; 13. P. sycoiuetra. 



I. IX. Artificial system with a narrower extension of the idea of species 

 (in the third degree) : 7 genera with 39 species. The genera would be : — 

 1. Doruyrantiu ; 2. Cystoyrantia ; 3. Connoyrantia ; 4. C(xnoyra7itia ; 

 5. Tarroyruntia ; G. Cophoyrantia ; 7. Metroyrantia. The 39 species would 

 be represented by the 39 forms which are cited as genera in the artificial 

 system in the second volume. Thus, for example, the second genus 

 (Cystoyrantia) would contain three species: — 1. C. clistolytithm ; 2. C. 

 lipostomella ; and 3. C. sycocystis. 



K. X. Artificial system with a stiU narrower extension of the idea of 

 species (in tlie fourth degree) : 7 families, with 19 genera and 181 species. 

 The 7 genera of the ninth system are here raised to the rank of families, 

 and the 19 species of the eighth system to that of genera; and the 181 

 species are the same that in the fifth system were divided into 43 essen- 

 tially distinct genera. 



h.Xl. Artificial system with an average extension of the idea of species 

 (in the fifth degree) : 7 orders, with 19 families, 39 genera, and 289 

 species. This system is carried out in the second section of the second 

 volume on the principles hitherto followed in the classification of sponges. 



M. XII. Artificial system witli a very narrow extension of the idea of 

 species (in the sixth degree) : 7 orders, with 19 families, 113 genera, and 

 691 species. Hero those groups of forms are regarded as genera which 

 in the eleventh system had only the rank of subg(>nera ( Olyuthettus, P>ys- 

 sycettus, Sycurettus, &c.), and as species those forms which figure in the 

 eleventh system as subspecies. 



