PLANKTONIC STUDIES. 637 



tUe problems in hand iiiiu-U too coinplicated. >ratliematical treatment 

 of these does more liarm tlian good, betuiuse it tjives a deceptive sem- 

 l)lauce of accuracy, which in fact is not attainable.* A part of physi- 

 ology also embraces such subjects as are with difficulty, or even not at 

 all, accessible to exact definition, and to these also belong the chorology 

 and (ecology of the plankton. 



The funda mental fault of Tlenseii^^ plankton tJieori/ in my opinion lies 

 in the fact that he regards a highly complicated i)roblem of biology as 

 a relatively simple one, that he regards its many oscillating- parts as 

 pro])ortlonally constant bulks, and that he believes that a knowledge 

 of these can be reached by the exact method of mathematical counting 

 and computation. This error is partly excusable from the circum- 

 stance that the i»hysiology of to-day, in its one-sided pursuit of exact 

 research, has lost sight of many general problems which are not suited 

 for exact special investigation. This is shown esi)ecially in the case 

 of the most important question of our i)resent theory of develop- 

 ment, the species jjroblem. The discussions which Ilensen gives 

 upon the nature of tlie species, u})on systemization, Darwinism, and 

 the descent theory, in many places in his plankton volume (pp. 19, 41, 

 7o, etc) are among the most peculiar v/hich the volume contains. They 

 deserve the special attention of the systematist. The "actual species" 

 is for him a physiological conception, while, as is known, all distinction 

 of species has hitherto been reached by morphological means.t 



In my Report on the Badiolaria of H, M. S. Challenger I have at- 

 tempted to point out how the extremely manifold forms of this most 

 numerous class (739 genera and 4,.'U8 species) are on the one hand dis- 

 tinguished as species by morphological characters, and yet on the other 

 hand may be regarded as modifications of 85 family types, or as de- 

 scendants of 20 ancestral orders, and these again as derived from one 

 common simple ancestral form {Actis.saj 4, § loS). Hensen on the other 

 hand is of the opinion that therein is to be found "a strong opposing 

 proof against the independence of the species" (9, j). 100). He hopes 

 "to lighten the systematic difiiculties by the help of computation" (p. 

 75). Through his systematic plankton investigations he has reached 



* A familiar and very iustrtictive t'xauii)l(i of this perverted employment of exact 

 methods in morphblogy is furnislied by the familiar "Mechanical theory of develop- 

 ment" of His, which I have examined in my anthropogeny (3d edition, p. 53, 655) as 

 well as in my paper upon Ziclc and Wege dcr Entwickehingsgeschichte (Jena, 1875). 



t Since of late the physiological importance of the "species" concci)tiou has often 

 been emphasized and the " system of the futnre " by the way of '•' comparative physi- 

 ology " has been pointed out, it must here be considered that np to this time not 

 one of these systematic physiologists has given even a hint how this new system of 

 description of species can be practically carried out. What Hensen has said about 

 it (i», pp. 41, 73, 100) is just as worthless as the earlier discussions by Pol^jaeff, which 

 have been critically considered in my Report on the Deep-Sea Kerafcosa {Challenger, 

 Zoology, vol. XXXII, part 82, pp. 82-85.) 



