TRANSITIONS IN MORPHOLOGICAL CHARACTERS 123 



an increase in vitality, but rather, that it is an indifferent though 

 a vacillating, rhythmic process leading to retrogressive as well 

 as progressive changes. 



In addition to this meager amount of experimentation certain 

 observations have been made upon variation in nature and cer- 

 tain theories have been brought forward to interpret them which 

 here deserve consideration. 



L. A. Jagerskiold (94) has observed new pelagic varieties of 

 the rotifers Anuraea cochlearis and A, aculeata in the Baltic 

 Sea. The substance of his explanations of these varieties are 

 contained in the following quotations. 



Wahrscheinlich ist wohl, dass diese ich mochte fast sagen Tendenz, 

 mit gekriimmten und auswarts gebogenen Hornern zu variieren, die bei 

 den oben beschriebenen Formen vorhanden ist, auf das pelagische 

 Leben dieser Tiere zuriickgefiihrt werden mus;? und als gewissermassen 

 mit dem Analog zu bezeichnen ist, Avas man bei den pelagischen Dia- 

 tomaceen gefunden hat. 



In addition, Jagerskiold has found intermediate forms be- 

 tween the typical species of Anuraea and these varieties. 



We would call attention to the indefiniteness of ascribing these 

 varieties to pelagic life as a whole without effort to analyse its 

 factors; and we raise the question whether it is not possible that 

 the variation is here again due to the food changes which would 

 all but inevitably follow from the transfer from fresh to salt 

 water. 



On this subject of variation in Anuraea cochlearis another 

 scientist, R. Lauterborn, has made extensive observations. In 

 an article pubhshed in 1904 on ''Die cyklische oder temporale 

 Variation von Anuraea cochlearis," he considers the many 

 variations of this rotifer as closelj'' related to changes in tempera- 

 ture. In the spring and summer months types are present which 

 are entirely absent during the winter months. On page 589, he 

 says: 



Wir wenden uns nun zu einer speziellen Betrachtung derjenigen 

 Gewasser, in welchen A. cochlearis durch die Tecta-, Hispeda-, und 

 Irregularis-Reihen vertreten ist. Hier trat uns schon friiher wiederholt 

 der bestimmende Einfluss vor Augen, den der jahrliche Gang der Temper- 

 atur auf die Grosse des Panzers der Gesamtart ausiibt. Wir diirfen 



