PLANKTONIC STUDIES. 639 



regards linuger as the cause of tliis, and the "tendency to explore a 

 relatively great l)iilk of water." In general, according to liis view, 

 " many larger pelagic animals bear tlie outspoken character of unfavor- 

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



Regarding the appearanci' of nuiny pelagic animals in sirarnis, riensen 

 explains " that the young do not tioat, but swim freely. In conseipience 

 of this, the mother animals drive them away, and if the larv.e finally 

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

 them" (23, p. 252). The siccuniulation of numbers oIl Pliijf<ali(( in great 

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

 movement. The animals which are capable of luj independent move- 

 ment of progression must renuiin rather closely crowded together, iu 

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

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

 not, as Hensen assumes, gonochoristic, but always hcrmaphrodiUc,* 



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 iu the zoocurrents (p. 85), particu- 

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

 lems which planktology furnishes for human investigative energy. For 

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

 establishment of the descent theory, has opened to its 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 hereditj^ and variation, iu order to learn to 

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

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

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

 employed in a manner worthy of imitation: sinmltaneous "observation 

 and reflection." 



* The coriui of all rhijsaUdw are mon«'cions, their cormidia moiioclinic. Each 

 single hranch of the racemoso youodcndroii is nionostylic, and hears one female and 

 several male mednsoids. Tlio facts were brought out thirty-five years ago by Huxley. 

 (Compare my Report on the Siphonophorie : Zoology of the ChaUeiHjcr, vol. xxviii, 

 pp. 347, 35G.) 



