230 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



with thick walls and coarse pores, and those with thin walls and fine 

 pores in a different pattern of arrangement. In such chains cells were 

 also found with the two valves different, one of them of the heavy, the 

 other of the lighter type. The cells of the two types were, however, of 

 the same form and size. He concluded in his first interpretation (1903, 

 1904) that he was dealing with instances of " Sprnngweise Mutation." 

 These mutations occurred in several species. Later (1906), following the 

 publication of Gran's paper ( 1904) and Karsten's (1904) critique upon 

 his own and Gran's interpretation, he changed his conclusion to accord 

 with that of Gran, that is, that these were cases of polymorphism. 



In the Arctic plankton Gran (1904) found Rhizosolenia semispina, a 

 species with thin walls, long intermediate bands and slender pointed 

 tips with a thin terminal bristle, a species abundant in spring and sum- 

 mer. Within its range of distribution occurs also R. hebetata, a winter 

 species of about the same size, with thick walls, short intermediate bands, 

 and blunt rounded tips. Chains are not found in these species, but 

 Gran found single individuals in which one half of the skeleton was that 

 of R. hebetata and the other that of R. semispina, indicating a sudden 

 change from the former to the latter. A similar connection between 

 two species was found by Gran in chains containing Chaetoceras criophi- 

 Jum and C. boreale, in which the former has suddenly given rise to the 

 latter. It is, perhaps, significant that these mutating chains were taken 

 "aus relativ warmen Wasser ausserhalb des nordlichen Norwegens," 

 while the Rhizosolenia in which evidences of mutation were found oc- 

 curred in " Planktonmaterial von der Grenze des Treibeises." These 

 are both regions where environmental contrasts were strongly marked, 

 where factors exist which might call forth mutations. 



Gran's interpretation is best given in his own words : " Wenn um die 

 Hhizosolenien als Beispiel zu nehmen — R. hebetata sich in R. semisjyina 

 verwandeln kann, aber R. semispina niemals oder nur ausnahmsweise in 

 R. hebetata ubergeht, dann konnen wir von einer Mutation sprechen. 

 Wenn aber die beiden Formen unter dem Einfiuss von wechselnden aus- 

 seren Faktoren regelmassig ineinander iibergehen konnen, dann haben wir 

 eigentlich nur eine Art, und die Veranderungen sind Aeusserungen eines 

 gesetzmassigen Dimorphismus." He adds thereto the idea that in the 

 two species with practically similar areas of distribution Rhizosolenia hebe- 

 tata is adapted to winter and Arctic conditions with slow reproduction, 

 and R. semispina to those of spring and summer with more rapid multipli- 

 cation and greater competition. He notes also the points of structure in 

 which the winter form R. hebetata approaches a diatom resting spore, a 



