18 GENERAL EVOLUTION". 



side, on the outer surface. In Goniatites a fold is added on the 

 back. In successive genera other main folds are added, the last 

 appearing nearest the center of the coil. These then become 

 complicated by subordinate undulations which in the more com- 

 plex forms become the axes of a double row of new lobes and folds, 

 the whole presenting symmetrical lobate outlines of much com- 

 plexity and beauty. 



But another series of changes accompanies those of the septa, 

 and are entirely independent of them. These relate to the yari- 

 ous degrees of winding of the shell. The early form with simple 

 septa (Orthoceras) was straight, but others which succeeded began 

 to turn their shell-making round an axis, thus commencing a par- 

 tial coil. In some the coil was very open ; in others it began 

 close, and then ceased, the shell finishing straight. Others after 

 such a course began to wind again, while some made a single turn 

 near the middle. The most remote from -the starting-point [A^n- 

 monites, sp.) made a tight and complete flat coil, while some, 

 whose septa remained simple, did the same (JVautilus). 



The direct uses of these various forms of septum and coil are 

 simply inexplicable, and that one of them was any *^ fitter" to 

 *^ survive" than another, by virtue of its usefulness, is for me 

 more than doubtful. 



I am tempted to continue this theme, for it might be pro- 

 longed indefinitely by any one familiar with the details of ana- 

 tomical structure, but I will only repeat that the illustrations 

 would be drawn chiefly from the characters of the classes, orders, 

 families, and other higher groups. 



It is not difficult to believe, in the case of the useful structures 

 first cited, that the law of natural selection has had much, prob- 

 ably everything, to do with the preservation of tbe animals pos- 

 sessing them in the various localities to which they are adapted. 

 But that it has had opportunity to direct the lines of progress in 

 the second series is not likely. That it had nothing to do with 

 the origin of either, is certain. 



4. On Groivtli- Force. 



Every change by complication of structure is by addition; 

 every simplification is by subtraction. Every addition is a matter 

 of growth, which is carried on by a process of nutrition. The in- 

 quiry respecting the origin of new forms centers itself at once on 

 the history of growth and the influences affecting it. 



