NATURAL HISTORY OF THE DIATOMACE7E. 441 



It will be naturally supposed, from what has been said on the subject, 

 that the mode in which the motion is caused is not decidedly known ; 

 but however that may be, it is a remarkable characteristic of these crea- 

 tures, and, although in most species it is regular and uniform, in some it 

 takes place as a series of jerks. It would seem that it could hardly be 

 called a voluntary motion, or, at least, that if it be so, the faculty govern- 

 ing and directing it within the creature is of a low order, or one which 

 responds to stimulus in a very sluggish manner ; for if an obstacle of any 

 kind occur in the path of a moving diatom, like a Navicula, it is not 

 avoided ; but, on the contrary, if it be small enough, it is thrust aside ; 

 or, if it be too large, the diatom is arrested in its career for a certain 

 length of time, or turned aside in its course. If it be stopped, it is a 

 remarkable fact, noted by English observers, that the diatom waits exactly 

 the length of time it would have taken for it to perfect its forward pro- 

 gression to the greatest extent, when it returns on its path again. In 

 many species, however, the motion is not so regular as this, and the little 

 creature goes tottering along its way. It has been remarked by one 

 author that "the movements of the diatomaceae appear rapid and viva- 

 cious under the microscope; but it must be remembered that the high 

 powers usually employed in the observation of these minute organisms 

 magnify their motions as well as their bulk." Using a seconds watch, 

 and timing several species exactly, it has been found that one of the 

 most rapid, known as Bacillaria paradoxa, took a whole second to move 

 over one two-hundreth of an inch ; and that one of the slowest, named 

 Pinnularia radiosa, in the same space of time only travelled one thirty- 

 four-hundredth of an inch ; so that the quickest would take three min- 

 utes to travel an inch, while its slowest relative would require a full hour 

 to perform the same feat. But although a few observations of this kind 

 have been made, we have by no means arrived at a knowledge of the 

 rate of movement of these atomies, for it varies under different circum- 

 stances, as apparent condition of health and surroundings. Heat appears 

 to accelerate as cold retards it ; and yet I have seen Bacillaria paradoxa 

 very lively when taken from beneath the ice on a cold winter's day. It 

 is a curious fact, that often, as we watch a diatom sailing across the field 

 of the microscope, it comes in contact with an obstacle, as a grain of 

 sand. If it cannot move it, or pass under it, or, by a little shifting, around 

 VOL. i. 58 



