246 bulletin: museum of comparative zoology. 



vertical distribution of certain species. Yet the differences in the envi- 

 roning factors attending these changes in distribution may be very slight. 

 The pelagic fauna is attuned to environmental changes of smaller 

 amplitude than that to which the shore and land fauna are subjected, 

 and presumably is in turn profoundly affected by changes in these factors 

 of an amplitude which has little or no influence upon organisms of a 

 fauna with more varied environment. 



My grounds for proposing this interpretation are twofold : — 



1. The occurrence of degeneration forms in flagellates under culture 

 in media different from their normal habitat. Ktlster (1908) finds that 

 Gymnodinium fucorum when cultivated in 1-2 per cent salt solution or 

 in sugar solutions gives rise to heteromorphic cysts of irregular forms 

 which may be designated as degeneration stages, in the narrow sense of 

 the word, since the forms are irregular and of various shape and the 

 cultures often die out. Amoeboid forms arise in agar and in gelatine 

 cultures. Zumstein (1900) was able to cultivate Euglena gracilis on 

 solid media and produce a heteromorphosis to Palmella-like groups of 

 organisms. 



The most striking instances, however, of heteromorphic changes are 

 those called forth in Trypanosoma under culture in blood agar. A 

 summary from the biological point of view of the recent results of work 

 in this line will be found in Doflein (1909). Trypanosoma in such 

 cultures takes on the form of Herpetomonas, an intestinal parasite of 

 Diptera, a transformation which involves a considerable change in the 

 position and relations of the tlagella. The process is, moreover, reversible, 

 for the Herpetomonas forms reintroduced into the blood are changed 

 again to Trypanosoma. The heteromorphoses here induced by the 

 modified environment are significant of the probable path of evolution 

 of the parasitic Trypanosoma and might be regarded as an atavistic 

 reversion. These reversions and the degeneration of Trypanosoma arising 

 in other cultures may be regarded as stages in the operation of the same 

 heteromorphic process in which the organism takes on another, and 

 perhaps in some instances an ancestral, type of structure. 



The heteromorphic chains of Ceratium may then represent a similar 

 biological process. There is in G. californiense and in the heteromorphs 

 figured by Lohmann a reduction in the extent of the horns. The parent 

 cell in some cases also undergoes autotomy which may be likewise 

 regarded as a degeneration phenomenon arising, as I have elsewhere 

 suggested (1908), in response to the change involved in the sinking 

 of the organism to deeper, colder waters. The heteromorphs possihly 



