342 Evolution and Adaptation 
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 are 
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 
