and the Theory of Descendence. 423 



the conditions of inheritance and adaptation may be unusually 

 clearly reviewed in the Calcispongi^. The part taken by these 

 two foimative functions in the production of the individual form 

 may be here determined more accurately and certainly than is 

 usually the case. 



2. Idea and Descendence of Species. 



The idea of the species is the central point of attack of the 

 theory of descendence, and the true nucleus of all discussions on 

 " development or creation." To investigate this idea again here 

 would be completely superfluous. I have explained my views 

 upon it in such detail in my criticism of the morphological, phy- 

 siological, and genealogical idea of species in my ' General Mor- 

 phology' (Bd. ii. pp. 323-364) that I should merely have to 

 repeat what I have there said. All attempts up to this time to 

 give the idea of the species a decided limit and contents have 

 failed, and by this negative result itself have led to the conviction 

 that the positive idea sought for cannot be defined. The 

 genealogical definition of the idea attempted by me is just as 

 unsatisfactory and untenable as all the rest. This lies in the 

 nature of the thing. The species is just as arbitrary an abs- 

 traction produced by the subjective contemplation of the author, 

 just as much a category of only relative significance, as the 

 ideas of the variety, genus, family, &c. All these categories 

 have their value only in their reciprocal relations to one another, 

 and owe their origin to the subjective law of specification (/. c. 

 p. 331). 



Moreover we have only to glance at the practice in zoological 

 and botanical classification to be convinced that the practical 

 distinction of species lias nothing at all to do with all these theo- 

 retical definitions of the idea of species. On the contrary there 

 prevails in that distinction the greatest subjective arbitrariness, 

 and hence an endless dispute between the various systematists. 

 No two systematists, who have thoroughly worked upon the 

 same group of forms, have ever yet agreed perfectly as to the 

 number and limitation of the species united in it. 



In the Calcispongige the practical distinction of species is 

 subject to much greater difficulties than in most other groups 

 of animals. According as the systematist conceives the idea 

 of the species in a wider or narrower form, according as he 

 estimates most highly the principles of the artificial or the 

 natural system, he may considerably increase or diminish the 

 number of 21 genera and 111 species of tlie natural system 

 which are described in the first section of my second volume. 

 The natural system might, for example, be founded upon any 

 one of the following six conceptions : — A. 1 genus with 1 



