336 ORGANIC GROWTH 



cell. At a later stage the functions of the cells diverge, while 

 the cells themselves develop specific forms. Evidently touch, 

 smell, and taste continue longest to be perceived by the same 

 cells, and for this reason we look in vain in many of the 

 lower multicellular animals for separate organs for these 

 sensations. 



The sensory stimuli, as various stages of one and the same 

 stimulus of motion, have, according to my view, been the 

 ultimate cause of the origin of various specific sense organs, 

 a view I previously expressed with special reference to eyes. 



This evolution of the characters of sense-cells could not 

 possibly have been produced by sexual mixture and selection, 

 by variation of the germ-cells although I do not deny to 

 the first two processes a share in the accomplishment of the 

 result. Not variations of the germ-cells which occurred by 

 chance in a definite direction, but a definite capacity of 

 modification in the protoplasm of the ectoderm the property 

 of the latter of becoming altered in a definite way under 

 particular stimuli has determined the modification. 



The best evidence of the truth of this, and at the same 

 time of the effect of use, lies in the fact that the higher sense 

 organs have always developed on the parts of the body best 

 adapted for the reception of the respective stimuli, and in 

 different animals on different parts, while the larvae had 

 originally an ectoderm all of one kind, as the lower multi- 

 cellular animals have in their adult condition. 



A proof, moreover, that the organism under the action of 

 external influences can only undergo particular definite modifi- 

 cations, that it can only yield to external demands by modifi- 

 cation in a definite and limited degree, lies, as I have already 

 urged, in the^'Medusse," 1 in the fact that the higher sense-organs 



1 P. 220, seq. I refer here to my description of the auditory organ of the 

 Medusa Carmarina as compared with the ordinary structure of the auditory organ 

 of the cycloneurous Medusae and many worms. Cf. my previously cited address 

 to the Naturalists' Congress at Munich, 1877 ; also Die Medusen, p. 222. 



