Comparative Physiology. 207 



" Some zoologists divide the animal kingdom into three 

 groups, Vertebrata, Annulosa, and Mollusca; plants would 

 then be Exogens, Endogens, and Acrogens ; the Mollusca in- 

 cluding Mollusca, Radiata and Phytozoa, as Acrogens do Ferns, 

 Fungi and Protophyta. 



" In the foregoing arrangement, what has been called the 

 circular system has, to a certain extent, been adopted. From 

 whatever point we start, we may, by pursuing the various gra- 

 dations of structure presented to us, return again to the same 

 point. This is characteristic of a natural group, which never 

 ends abruptly, beginning with the highest and ending with 

 the lowest. The typical member of a grouj) is that which ex- 

 hibits its peculiarities of form, structure, and economy in the 

 greatest perfection ; the types of the different groups being 

 always more widely asunder than the aberrant members which 

 connect them. It may be doubted whether the circular arrange- 

 ment is competent to express all the affinities of natural grouj)s ; 

 it is probably better represented by a sphere than a circle ; its 

 typical form being the centre, and the aberrant members con- 

 nected by affinity in all directions. We constantly meet with 

 exemplifications of the circular form in the groups both of plants 

 and animals; but its universality has by no means been esta- 

 blished ; much less can the number of divisions iu each circle be 

 restricted to five, as proposed by some." 



On the Symmetry of Organised Structures, he remarks, " that 

 besides the uniformity of the parts composing the two sides of 

 the body in the external form of animals, or bilateral symmetry, 

 there is a symmetry also in the regular arrangement of many 

 similar parts around a common centre, as in the Radiata, or in 

 a spiral disposition of similar organs around a cylinder, which 

 is the type of symmetry in the vegetable kingdom. In the simpler 

 algaj, lichens, and fungi, the growth is so modified by circum- 

 stances that it is impossible to assign determinate bounds to 

 the outline ; and hasty attemjDts to characterise the races by 

 external form have led to much erroneous multiplication of 

 species. In the higher fungi we find the radiated form. In 

 the mosses the leaves begin to be spirally arranged round the 

 axis ; and the spiral arrangement is more conspicuous in 

 ferns. In the Phanerogamia it may be regarded as the general 

 law of the arrangement of the branches, leaves, and parts of 

 the flower, that they are disposed spirally around the axis 

 of growth, though the proof miglit sometimes be difficult 

 from perturbing causes. Opposite and whorled leaves will be 

 rendered spiral or alternate by any cause which gives full de- 

 velopement to the stem. The circular arrangement of the parts 

 of the flower also arises from the non-developement of the axis 

 of the stem. The spiral is evidently formed by the union of a 



p 9. 



