PLANKTONIC STUDIES. 637 
the problems in hand much too complicated. Mathematical treatment 
of these does more harm than good, because it gives a deceptive sem- 
blance of accuracy, which in fact is not attainable.* A partof 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 cecology of the plankton. 
The fundamental fault of Hensen’s plankton theory in my opinion lies 
in the fact that he regards a highly complicated problem of biology as 
a relatively simple 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 nethod of mathematical counting 
and computation. This error is partly excusable from the eircum- 
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 Hensen gives 
upon the nature of the species, upon systemization, Darwinism, and 
the descent theory, in many places in his plankton volume (pp. 19, 41, 
73, ete.) are among the most peculiar which 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 Radiolaria 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 4,518 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 55 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, p. 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 instructive example of this perverted employment of exact 
methods in morphology is furnished 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 Ziele and Wege der Entwickelungsgeschichte (Jena, 1875). 
_ tSince of late the physiological importance of the “species” conception has often 
been emphasized and the ‘‘ system of the future ” by the way of ‘“‘ comparative physi- 
- ology” has been pointed out, it must here be considered that up 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 (9, pp. 41, 73, 100) is just as worthless as the earlier discussions by Poléjdéeff, which 
have been critically considered in my Report on the Deep-Sea Keratosa (Chullenger, 
Zodlogy, vol. XXXII, part 82, pp. 82-85.) 
