876 



SCIENCE 



[N. S. Vol. XXXVII. No. 962 



ward movement becomes slower and the Euglenm 

 begin to swerve farther than usual toward the 

 dorsal side. Thus the spiral path becomes wider 

 and the anterior end swings about in a larger 

 circle and is pointed successively in many different 

 directions. In some part of its swinging in a circle 

 the anterior end of course becomes directed more 

 nearly toward the light; thereupon the amount of 

 swinging decreases, so that the Euglena tends to 

 retain a certain position so reached. In other 

 parts of the swinging in a circle the anterior end 

 becomes less exposed to the light; thereupon the 

 swaying increases, so that the organism does not 

 retain this position but swings to another. The 

 result is that in its spiral course it successively 

 swerves strongly toward the source of light, then 

 slightly away from it, until by a continuation of 

 this process the anterior end is directed toward the 

 light. In this position it swims forward. 



Figs. 91 and 92, p. 135, show variations in 

 the severity of the reaction, the second figure 

 representing but a very slight widening of the 

 narrow spiral in which the organism has been 

 swimming. Fig. 93, p. 139, represents the 

 path of a Euglena executing a turn of ISO" 

 by a series of similar slight widenings of the 

 spiral. 



From such evidence it would seem that the 

 motor reflexes of Eugleiia appear in varying 

 degrees that shade more or less gradually into 

 each other as the strength of stimulation 

 varies. This admirably meets the require- 

 ments of a " tropism theory " that is expected 

 to account for the gradual but definite and 

 errorless turning movements executed by so 

 many bilaterally symmetrical organisms in 

 orienting themselves with respect to a source 

 of light. 



These considerations inclined me to the view 

 that in bilaterally symmetrical organisms the 

 shock reactions that have no obvious connec- 

 tion with orientation to a stimulus and are 

 produced by sudden changes in intensity of 

 light may occupy one end of a series at the 

 other end of which are the very small reactions 

 by means of which the tropic turning move- 

 ment is achieved. In that case the difference 

 in effect on orientation of these extreme cases 

 would not indicate any fundamental difference 

 in mechanisms governing them, but rather a 



pronounced difference in the magnitude of the 

 responses to stimuli of different intensities. 



Recently, however, my attention has been 

 called to new evidence, shortly to be published 

 by my friend, Dr. F. W. Bancroft, that in 

 Euglena the mechanisms of the shock reaction 

 and the tropic reaction are distinct. How gen- 

 eral this observation may prove to be is not 

 now certain. But in any case, the shock reac- 

 tion can hardly be said to occupy the position 

 of a prototype from which trialless heliotropic 

 turning movements have been derived by any 

 process of selection. 



Harry Beal Torrey 



Eeed College, 

 Portland, Oregon, 

 January 9, 1913 



AN AID TO STUDENTS 

 The Academy of Natural Sciences of Phil- 

 adelphia has published, as part of the after- 

 math of the brilliant centenary celebration of 

 last year, an index to its publications from 

 the first volume of the Journal issued in 1817 

 to the conclusion of the sixty-second volume 

 of the Proceedings completed in 1911,/ making 

 a total of eighty-three volumes. The portly 

 index comprises 1,433 octavo pages and is 

 divided into two sections. The first contains 

 the titles of all the contributions to the 

 series, arranged alphabetically under the 

 names of the authors, and ranges from brief 

 paragraph reports of the communications 

 made verbally before the meetings of the 

 academy to the classic quarto volume by 

 Joseph Leidy on the extinct manmialian 

 fauna of Dakota and Nebraska, and the beau- 

 tiful monographs on the burial mounds of the 

 south by Clarence B. Moore. 



The second section is composed of an alpha- 

 betical arrangement, from aalensis to Zythia, 

 of the names of every species, genus, and fam- 

 ily described or referred to in the several 

 volumes. It is estimated that there are about 

 124,600 such entries in the list and some idea 

 of the labor involved in its preparation and ar- 

 rangement may be had from the fact that the 

 original entries under the letter P numbered 

 19,500, under S 16,650 and under T 10,300. 



