THE METHOD OF CREATION OF ORGANIC FORMS. 181 



nutrition, or of the construction of organs and tissues out of pro- 

 toplasm. 



The construction of the animal types may be referred to two 

 kinds of increase— the addition of identical segments and the addi- 

 tion of identical cells. The first is probably to be referred to the 

 last, but the laws which give rise to it can not now be explained. 

 Certain it is that segmentation is not only produced by addition of 

 identical parts, but also by subdivision of a homogeneous part. In 

 reducing the vertebrate or most complex animal to its simplest 

 expression, we find that all its specialized parts are but modifica- 

 tions of the segment, either simply or as sub-segments of compound 

 but identical segments. Gegenbaur has pointed out that the most 

 complex limb with hand or foot is constructed, first, of a single 

 longitudinal series of identical segments, from each of which a 

 similar segment diverges, the whole forming parallel series, not only 

 in the oblique transverse, but generally in the longitudinal sense. 

 Thus, the limb of the Lepidosiren represents the simple type, that 

 of the Ichthyosaurus a modification. In the latter, the first seg- 

 ment only (femur or humerus) is specialized, the other pieces being 

 undistinguishable. In the Plesiosaurian paddle the separate parts 

 are distinguished, the ulna and radius well marked, the carj^al 

 pieces hexagonal, the phalanges defined, etc. 



As regards the whole skeleton, the same position may be safely 

 assumed. Though Huxley may reject Owen's theory of the verte- 

 brate character 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 entirely 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 mav be used. Cer- 

 tain it is, however, that the parts of the segments of the cranium 

 may be now more or less completely parallelized or homologized 

 with each other, and that, as we descend the scale of vertebrated 

 animals, the resemblance of these segments to vertebrae increases, 

 and the constituent segments of each become more similar. In the 

 types, as Amphioxus, etc., where the greatest resemblance is seen, 

 segmentation of either is incomplete, for they retain the original 

 non-osseous basis. Other animals which present cavities or parts 

 of a solid support are still more easily reduced to a simple basis of 

 segments, arranged either longitudinally (worm) or centrifugally 

 (star-fish, etc.). 



