FORMS OF LIFE 



from the under side travels to the upper side, and the 

 two eyes Ue together on one side (tlie right or left); 

 while the bones of the skull and the softer parts of each 

 side of the head grow quite crooked. Naturally, this 

 ontogenetic process, in which a striking lack of symmetry 

 succeeds to the early complete symmetry of each indi- 

 vidual, can only be explained by our biogenetic law; it 

 is a rapid and brief recapitulation (determined by 

 heredity) of the long and slow phyletic process which 

 the flat-fish has undergone for thousands of years in its 

 ancestral history to bring about its gradual modification. 

 At the same time, this interesting metamorphosis of the 

 pleuroncctidcs gives us an excellent instance of the 

 inheritance of acquired characteristics, as a consequence 

 of constant oecological habit. It is quite impossible to 

 explain it on Weismann's theory of the germ-plasm. 



We have another striking example among the inverte- 

 brates in the snails {gasteropoda). The great majority 

 of these moUusks 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 moUusk 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-symmet- 

 rical moUusk, by the two sides of the body having an 

 unequal growth. The cause of it was a purely me- 

 chanical 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 stronglv on one side than the other in conse- 

 quence of this; and this was accompanied by consider- 

 able displacement and modification of the neighboring 

 parts, especially the gills. In most snails one of the 



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