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years the latter is more abundant, and in two, the former. The 

 relative abundance of these forms in the river at a given point of 

 collection is an epitome of their distribution in a wide area of channel 

 and backwaters. An application of the principle advanced by 

 Zacharias would in this instance lead to constant change. The 

 retention of pala (Ehrbg., 1830) as the type and amphiceros (Ehrbg., 

 1838) as the variety is in keeping with priority in nomenclature and 

 with the principle of regarding the more highly differentiated or 

 divergent form as the variety. Variety amphiceros occupies thus 

 the same relation to the type that bidens does to its type angularis. 

 Both are illustrations of the tendency common to all species of 

 Brachionus to develop posteriorly directed spines. 



I shall proceed to discuss the salient points in the seasonal dis- 

 tribution and statistics of the several varieties : 



Brachionus pala Ehrbg., type. Average number of individuals, 

 2,693 ; of eggs, 20,809, including all free eggs referable to the species 

 in the broader sense. In the present connection I shall call attention 

 only to the fact that the type form,without the posterior spines, is less 

 abundant during the midsummer interval than the spinous variety 

 amphiceros. This appears in Table I., and is to be found in the 

 records of years prior to 1898. A fuller comparison of the records 

 of the two forms will be made in the discussion of amphiceros. I 

 shall not discuss the recurrent pulses of this form or of amphiceros, 

 since as they dominate those of the species as a whole it would lead 

 to considerable repetition. The pulses of pala in the main (Table I.) 

 coincide in location with those of the species as a whole, and the 

 direction of movement of the seasonal curve of distribution is quite 

 similar, save in the fact that the amplitude of the pulses is less, and 

 that the differences in seasonal distribution between pala and 

 amphiceros modify the curve of each. 



The decisive evidence of sexual reproduction in the species in the 

 form of attached male and winter eggs is found repeatedly at times 

 of the major pulses. In some instances they appear during the rise 

 of the pulse. The autumnal pulse of 1895 will serve as an illustra- 

 tion of the character of these statistical data. (See following page.) 



This pulse is sustained much longer than usual, but it serves to 

 show the prevalence of parthenogenetic eggs during the rise of the 

 pulse, and the evidence of sexual reproduction during its progress. 

 In some other instances the number of free winter eggs after the 



