PLANKTONIC STUDIES. 637 



the piobleiiis in liaud imu-li too coiiiplicated. ^ratliematical treatment 

 of these does more liarm than good, because it gives a deceptive sem- 

 bhmce of accuracj', which in fact is not attainable.* A partof physi- 

 oh)gy also embraces such subjects as are with difiiculty, or even not at 

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

 and cecology of the plankton. 



The fundamental fault of Hensenh pfankton theory/ in my opinion lies 

 in the fact that he regards a highly complicated problem of biology as 

 a relatively simi)le one, that he regards its many oscillating parts as 

 proportionally 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 physiology 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 especially in the case 

 of the most important question of our present theory of develop- 

 ment, the species problem. The discussions which Henaen gives 

 upon the nature of the species, upon systemization, Darwinism, and 

 the descent theory, in many jdaces in his plankton volume (pp. ID, 41, 

 73, etc.) are among the most peculiai> which the volume contains. They 

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

 is for him a physiological conception, m hile, as is known, all distinction 

 of si)ecies 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 (7o9 genera and 4,318 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 {Actissa, 4, § 158). 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 difficulties by the help of computation" (p. 

 75). Through his systematic plankton investigations he has reached 



"^ A familiar and very instriictivo cxampl*) of this perverted eniployuicnt of exact 

 uietliods in morphology is furnisiied by the familiar "Mechanical theory of develop- 

 uieut" of Ilis, which I have examiued in my authropogcny (3d edition, p. 53, ()55) as 

 well as in my paper upon Ziele and U'cge dcr Entwickeliiiiijsgeschichle (Jena, 1875). 



tSinco of late tlie i)iiysiologieal importance of the "species" conception has often 

 been emphasized and the " system of the future " by the way of '•' comparative pliysi- 

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

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

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

 it (i), pp. 41, 73, 100) is just as worthless as the earlier discussions by Polejaeff, which 

 have been critically considered in my Report on the Deep-Sea Keratosa {CluilUnujer, 

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



