We have another striking example among the invertebrates in the snails (_gasteropoda_). The great majority of these mollusks are characterized by the spiral shape of their shells. This variously shaped, and often prettily colored and marked, snail's house is in essence a spirally coiled tube, closed at the upper end and open at the lower (or mouth): the mollusk can at any moment withdraw into its tube. The comparative anatomy and ontogeny of the snails teach us that this spiral shell came originally from a simple discoid or cylindrical dorsal covering of the once bilateral-symmetrical mollusk, by the two sides of the body having an unequal growth. The cause of it was a purely mechanical factor--the sinking of the growing visceral sac, covered with the shell, to one side; one part of the viscera contained in it (the heart, kidneys, liver, etc.) grew more strongly on one side than the other in consequence of this; and this was accompanied by considerable displacement and modification of the neighboring parts, especially the gills. In most snails one of the gills and kidneys and the ventricle of the heart corresponding 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 expression in the spiral form of the snail's shell. This remarkable 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 completely irregular and without axes, as usually the attraction 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. Nevertheless, we may instance as quite irregular the soft and ever-changing plasma-bodies of many rhizopods, the amœbinæ, mycetozoa, etc. Most of the sponges also--which we regard as stocks of gastræads--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 configurations 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 morphological transformation. But the question arises as to the origin of these few geometrically definable types, and the cause of their divergence.