342 Evolution and Adaptatioii 



examine its method of locomotion, we find that it moves 

 indifferently in any direction over a solid surface ; that is, 

 it keeps its oral face against a solid object, and moves over 

 the surface in any direction. Under these circumstances 

 the same external conditions will act equally upon all sides 

 of the body. In contrast to these common sea-urchins, 

 there are two other related groups, in which, although traces 

 of a well-marked radial symmetry are found, the external 

 form has been so changed that a secondary bilateral form 

 has been superimposed on it. These are the groups of the 

 clypeasters and the spatangoids, and it is generally supposed 

 that their forefathers were radially symmetrical forms like 

 the ordinary forms of sea-urchins. These bilateral forms 

 move in the direction of their plane of symmetry, but 

 we have no means of knowing whether they first became 

 bilateral and, in consequence, now move in the direction of 

 the median plane, or whether they acquired the habit of mov- 

 ing in one direction, and in consequence acquired a bilateral 

 symmetry. It seems more probable that the form changed 

 first, for otherwise it is difficult to see why a change of move- 

 ment in one direction should ever have taken place. 



The radially symmetrical form is characteristic of many 

 flowers that stand on the ends of their stalks. They also 

 will be subjected to similar external influences in all direc- 

 tions. Many flowers, on the other hand, are bilaterally 

 symmetrical. Some of these forms are of such a sort that 

 they are generally interpreted as having been acquired in 

 connection with the visits of insects. Be this as it may, 

 it is still not clear why, if the flowers are terminal, insects 

 should not approach them equally from every direction. If 

 the flowers are not terminal, as, in fact, many of them arc 

 not, their relation to the surroundings is bilateral with re- 

 spect to internal as well as to external conditions. The 

 former, rather than the latter, may have produced the 

 bilateral form of the flower. Here also we meet with the 



