120 MORPHJLUGiCAL PEVELOPMEXT. 



DiatoniacecBj of which Figs. 2 and 3 show examples, severally? 



iiickide genera characterized 

 ® {^ /^ ...^x!?,^ k ^y triple bilateral s}Tn- 



® 



metry. A Navicula is di- 

 visible into corresponding 

 halves by a transverse 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 transversely-constricted forms 

 of DesmidiacecB, exemplified by the second of the individuals 

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

 lated to its environment, we see that its mode of life exposes 

 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 mo^dng, 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 cor- 

 responding right and left sides, the s}mi metrical distribu- 

 tion of parts answers to the s}Tn metrical distribution of 

 forces. Passing to the two edges and the two flat sur- 

 faces, we similarly find a clue to their likenesses and differ- 

 ences in theii' respective relations to the things around them. 

 These locomotive protophytes move through the entangled 

 masses of fragments and fibres produced by decajdng organ- 

 isms and confervoid growths. The interstices in such matted 

 accumulations are nearly all of them much longer in one 

 dimension 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 ac- 

 quired by diatoms having this habit ; we also see that like- 



