102 R. J. ANDERSON [august 



seem odd that the two forms of prisms that the trimerous and penta- 

 merous symmetries suggest are asymmetric. The fact that arboraceous 

 monocotyledons dwell in the tropics, and that dicotyledons dwell in 

 temperate regions, has been commented on. The Dicksonias of New 

 Zealand and the araucarias of South America have chosen curious 

 places for homes. The sun in rotating on its axis, in sending its 

 rays through an atmosphere that partly polarizes the rays which are 

 going through the air with various degrees of obliquity, and the same 

 luminary in having its countenance affected by spots occasionally, not 

 to speak of the various wave whirls that may affect rays going in 

 different directions, may be held responsible for some of these dis- 

 crepancies. The rays, if they are of such a nature as to be alterable 

 by a crystal, may be naturally expected to have some power to alter 

 the character of a crystal, or other substance, and so a crystal may get 

 a molecular twist, and the plant that uses the crystal as food may 

 become similarly influenced, or get directly altered itself; but although 

 there may have been a tendency to molecular twisting in the young 

 plant by the sun's rays, grown plants are not so apparently affected ; 

 the plant tissues seem to have some power of correction, and so the 

 difference in the effects of the symmetry of the rays in the north 

 as compared with the southern hemisphere is not observed. 



The symmetry of animals is of various kinds. The spherical kind 

 is illustrated in the Protozoa. The Radiolaria, with their rays and 

 their trellis work, show us what was, or is, being done, and raise inquiry 

 as to the various agents that may be at work in bringing about 

 the result. Still water or some inert fluid may be looked upon as 

 favouring the maintenance of the spherical form seen in the resting 

 stages of many Protozoa, but the surface tension may also contribute 

 largely to the result. The sea anemones, simple sponges, and corals are 

 admirable examples of the modified cylindric symmetry ; the medusae 

 illustrate the modified spherical symmetry. The mouth in the centre 

 with appropriate radiating tubes, and in some cases the actual provision 

 of separate segments with a definite nervous system, shows a very 

 important departure in the bearings of the symmetry of a body on its 

 life. The welfare of many an animal is so much connected with its 

 colonial habits that its separation often means rapid extinction. The 

 chance of extinction is diminished by the segmentation in question. 

 Each part is, in a manner, independent of its neighbours ; so are the 

 parts of a star-fish, which may live after separation. A single ray may 

 even turn over. A mechanical advantage seems also to be derived from 

 a pair of fixed planes placed at right angles to one another, both as 

 regards purchase and security, in the case of certain medusoids. The 

 rhythm of Rhizostoma seems independent of the symmetry, 20 to 24 

 contractions per minute in a closed vessel were noted in one case. The 

 rhythm is best counted in the sea, however, an operation which is only 

 possible there in some medusae. The motion of the fluid from centre 



