THE GENERAL SHAPES OF PLANTS. 135 



verse plane and by two longitudinal planes one cutting its 

 valves at right angles and the other passing between its 

 valves. The like is true 

 of those numerous trans- 

 versely-constricted forms of 

 Desmidiacece, exemplified by 

 the second of the individuals 

 represented in Fig. 2. If now we ask how a Navicula is 

 related to its environment, we see that its mode of life ex- 

 poses it to three different sets of forces : each set being 

 resolvable into two equal and opposite sets. A Navicula 

 moves in the direction of its length, with either end foremost. 

 Hence, on the average, its ends are subject to like actions 

 from the agencies to which its motions subject it. Further, 

 either end while moving exposes its right and left sides to 

 amounts of influence which in the long run must be equal. 

 If, then, the two ends are not only like one another, but have 

 corresponding right and left sides, the symmetrical distribu- 

 tion of parts answers to the symmetrical distribution of 

 forces. Passing to the two edges and the two flat surfaces, 

 we similarly find a clue to their likenesses and differences in 

 their respective relations to the things around them. These 

 locomotive protophytes move through the entangled masses 

 of fragments and fibres produced by decaying organisms and 

 confervoid growths. The interstices in such matted accu- 

 mulations are nearly all of them much longer in one dimen- 

 sion than in the rest form crevices rather than regular 

 meshes. Hence, a small organism will have much greater 

 facility of insinuating itself through this debris, in which it 

 finds nutriment, if its transverse section is flattened instead 

 of square or circular. And while we see how, by survival of 

 the fittest, a flattened form is likely to be acquired by 

 diatoms having this habit; we also see that likeness will be 

 maintained between the two flat surfaces and between the 

 two edges. For, on the average, the relations of the two flat 

 surfaces to the sides of the openings through which the 



