THE WONDERS OF LIFE 



gills and kidneys and the ventricle of the heart corre- 

 sponding to these have disappeared altogether, only 

 those of the opposite side remaining; and the latter 

 have moved from the right side to the left, or vice versa. 

 The conspicuous lack of symmetry between the two 

 halves of the body which resulted from this finds ex- 

 pression in the spiral form of the snail's shell. This re- 

 markable ontogenetic metamorphosis also can be fully 

 explained by a corresponding phylogenetic process, and 

 affords a very fine instance of the inheritance of acquired 

 characters. 



There are also many examples of this asymmetry of 

 bilateral forms in the plant world, such as the green 

 foliage-leaves of the familiar begonia and the blooms of 

 canna. 



IV. The Centraporia. — Few organic forms are com- 

 pletely irregular and without axes, as usually the attrac- 

 tion to the earth {geotaxis) or to the nearest object 

 determines the special direction of growth, and so the 

 formation of an axis in some direction or other. Never- 

 theless, we may instance as quite irregular the soft and 

 ever-changing plasma-bodies of mai^y rhizopods, the 

 amoebinae, mycetozoa, etc. Most of the sponges also — 

 which we regard as stocks of gastraeads — are completely 

 irregular in structure; the most familiar example is the 

 common bath-sponge. 



An impartial and thorough study of organic forms has 

 convinced me that their actual, infinitely varied con- 

 figurations may all be reduced to the few typical forms 

 I have described. Comparative anatomy and ontogeny 

 further teach us that the countless modifying processes 

 which have led to the appearance of the various species 

 have acted by adaptation to different environments, 

 habits, and customs, and give us, in conjunction with 

 heredity, a physiological explanation of this morpho- 

 logical transformation. But the question arises as to 



iSo 



