PLANKTONIC STUDIES. 639 



regards Imiiger as the cause of this, uud the "teiuleucy to explore a 

 rehitively great bulk of water." In general, according to his view, 

 " many larger pelagic aninuils bear the outsi)oken character of unfavor- 

 able conditions of life, of a life of hunger." 



Eegarding the appearance of many i^elagic animals in swarms, Hensen 

 explains " that the young do not float, but swim freely. In conse(iuence 

 of this, the mother animals drive tirem away, and if the larva', linally 

 rise to the surface the former can not enter into competition with 

 them" (23, p. 252). The accumulation of numbers of FlujsaHa in great 

 swarms stands, according to his view, in correlation with the mode ot 

 movement. The animals which are capable of no independent move- 

 ment of progression must renmiu rather closely crowded together, in 

 order to be able to reproduce hisexnaJUj ; those carried too far away 

 must perish." On the other hand it is to be noted that the Physalia is 

 not, as Hensen assumes, go)iochoristic, but always hermaphroditic* 



The above-mentioned phenomena, the similarity to water of the pe- 

 lagic fauna, the periodic appearance of many pelagic organisms in 

 swarms, their abundant accumulation in the zoi (currents (p. 85), particu- 

 larly their relation to the currents, are only a few of the greater prob- 

 lems which planktology furnishes for human investigative energy. For 

 these, as for so many other fields of biology, Charles Darwin, by the 

 establishment of the descent theory, has opened to us the way to 

 a knowledge of causes. We must study the complicated reciprocal 

 relations of the organisms crowded together in the struggle for exist- 

 ence, the interaction of heredity and variation, in order to learn to 

 understand the life of the plankton. But in these plankton studies, as 

 well in physiological as in morphological questions, we nuist use that 

 method Avhich Johannes Miiller, the discoverer of this field, always 

 employed in a manner worthy of imitation: simultaneous "observation 

 and reflection." 



* The cormi of all Physalida; are moua-cions, their cormidia monoclinic. ICach 

 siu"-le brancli of the racemose gonodcudron is niunostylic, and hears one female and 

 several male medusoids. The facts were brought out thirty -five years ago by Huxley. 

 (Compare my Keport ou the SiphouophoriC : Zoology of the Challenger, vol. xxviii, 

 pp. 347, 356.) 



