454 FACTORS OF ORGANIC EVOLUTION. 



which I do not remember to have seen stated in the exact form in 

 which it Kes in my miud. 



Both lines probably started out with a more or less well-marked cir- 

 cular arrangement of the parts or organs. This was consequent upon 

 the peripheral arrangement of the new cells in the development of the 

 multicellular organism from the unicellular one. A long line of animal 

 life developed in obedience to this peripheral or rotate type of organi- 

 zation, ending in the echinoderms and some of the mollusks. This line 

 long ago reached its zenith. No line of descent can be traced from 

 them, according to Cope. The progressive and regnant type of animal 

 life appeared in the vermes or true worms, forms which are character- 

 ized by a two-sided or bilateral, and therefore more or less longitudinal, 

 structure. The animal-like organisms were strongly developed in the 

 power of locomotion, and it is easy to see that the rotate or centrifugal 

 construction would place the organism at a comparative disadvantage, 

 because its seat of sensation is farthest removed from the external 

 stimuli. But the worm-like organisms, "being longitudinal and bilat- 

 eral," writes Cope, ''■one extremity becomes differentiated by first con- 

 tact with the environment." In other words, the animal type has 

 shown a cephalic or head-forming evolution in consequence of the 

 bilateralism of structure. The individual has become concentrated. 

 Out of this worm-form type, therefore, all the higher ranges of zootypic 

 evolution have sprung, and one is almost tempted to read a literal 

 truth into David's lamentation that "I am a worm and no man." 



If, now, we turn to plants we find the rotate or peripheral arrange- 

 ment of parts emphasized in all the higher ranges of forms. The most 

 marked bilateralism in the plant world is among the bacteria, des- 

 mids, and the like, in which locomotion is markedly developed; and 

 these are also among the low^est plant types. But plants soon became 

 attached to the earth, or, as Cope terms them, they are " earth para- 

 sites." They therefore found it to their advantage to reach out in 

 every direction from their support in tlie search for food. Whilst the 

 centrifugal arrangement has strongly tended to disappear in the animal 

 creaition, it has tended with equal strength to persist and to augment 

 itself in the plant creation. Its marked development among plants 

 began with the acquirement of terrestrial life, and with the consequent 

 evolution of the asexual or sporophytic type of vegetation. Normally 

 the higher tyj)e of plant bears its parts more or less equally upon all 

 sides, and the limit to growth is still determined by the immediate 

 environment of the given individual or of its recent ancestors. Its 

 evolution has been acephalic, diffuse, or headless, and the individual 

 plant or tree has no proper concentration of parts. For the most part 

 it is filled with unspecialized plasma, which, when removed from the 

 parent individual (as in cuttings and grafts), is able to reproduce 

 another like individual. The arrangements of leaves, branches, the 

 parts of the flower, and even of seeds in the fruit, are thus rotate or 

 circular, and in the highest type of plants the annual lateral increments 



