II FIRST APPEARANCE OF NEW CHARACTERS 



61 



in complete harmony with my theory of growth but as very 

 clear evidence of its truth. They may be compared with 

 processes which can be recognised in the, so to speak, more 

 transparent individual growth of any plant. Such comparison 

 holds especially for the 2d and 4th of these facts ; plants 

 grow from the apex; and from the terminal point of the 

 evolution which has previously taken place, that is, of the 

 structure already formed, from the so-called vegetating point, 

 new characters arise. 



Thus the proper permanent leaves arise after the appear- 

 ance of the seed-leaves ; after the foliage-leaves, the flowers, 

 with their parts formed of modified leaves, and among the 

 foliage-leaves, again, sometimes various successive forms or 

 even colours. 



As in plants, the new characters show themselves also in 

 the shells of Ammonites at the youngest part, as it were at 

 the growing point. For in these shells, as in snail shells, the 

 parts near the aperture are the newest. The growth of 

 these shells takes place, as we may observe still in the shell 

 of the Nautilus, in an intermittent manner ; it becomes more 

 vigorous at each period of maximum vigour in the animal, 

 which probably coincides with the most favourable season of 

 the year, again to decrease and come to a standstill, just as 

 the growth wdiose results are seen so beautifully in the shells 

 of our vineyard snails, and the annual rings of any tree-trunk. 

 That this j)ei'ioclical growth results in the formation of 

 characters appearing successively more strongly or more 

 feebly is not to be wondered at. A high degree of symmetry 

 or metamerism may be produced by it. 



These considerations seem to me also to afford an explana- 

 tion of the origin of the symmetrically, i.e. metamerically 

 arranged characters of segmented animals, and to enable them 

 to be included in the same class of phenomena. That in such 

 segmented animals each somite should have the same 



