oe 
Cope. } 236 [Dee. 15, 
acter of the segments of the brain case, because they are so very different 
from the segments in other parts of the column, the question rests en- 
tively on the definition of a vertebra. If a vertebra be a segment of the 
skeleton, of course the brain case is composed of vertebrae; if not, then the 
cranium may be said to be formed of ‘‘sclerotomes,’’ or some other name 
may be used. Certain it is, however, that the parts of the segments of 
the cranium may be now more or less completely parallelised or homolo- 
gised with each other, and that as we descend the scale of vertebrated 
animals, the resemblance of these segments to vertebra increases, and the 
constituent segments of each become more similar. In the types Amphi- 
-oxus, etc., where the greatest resemblance is seen, segmentation of either 
is incomplete, for they retain the original cartilaginous basis. Other ani- 
mals which present cavities or parts of a solid support are still more 
easily reduced to a simple basis of segments, arranged either longitudin- 
ally (worm) or centrifugally (star-fish, etc.) 
DEFINITIONS. 
a The succession of construction of parts of a complex, was originally 
“a succession of identical repetitions; and grade influence merely deter- 
mined the number and location of such repetitions. 
f Acceleration signifies addition to the number of those repetitions 
during the period preceding maturity, as compared with the preceding 
generation, and retardation signifies a reduction of the numbers of such 
repetitions during the same time. 
7 The successive additions now characterizing the growth of the 
highest animals are not exact repetitions of segments at this time, be- 
cause of influences brought to bear on cell nutrition during long periods. 
The nature of these influences is made the subject of another section. 
Tn the endeavor to prove these positions, I will produce evidence, first, 
that some simpler animals grow according to the principle of modified 
repetitive addition, and that traces of it are to be observed in the most 
«complex ; second, that every addition to structure which has resulted in 
the complexity of the higher animals, was originally a repetition of a pre- 
existent structure. 
Detailed explanations of the law of repetitive addition are attempted 
in the following pages, under two heads, segment repetition, and.cell re- 
petition. 
A. ON SEGMENT REPETITION. 
This is everywhere seen in the construction of animals and plants 
Double bilateral symmetry may serve as one example of repetition in 
growth. 
a Bilateral symmetry. Anatomists have little difficulty in determin- 
ing the bilateral symmetry in most animals; that is, the homologies of 
the parts on opposite sides of the median line. It might be almost as- 
serted that it was a necessity of organization, but when we observe the 
growth of many plants, we are undeceived. And though bilateral symmetry 
in the Oelenterata and many Articulata is perfect, yet in higher animals it 
