HEREDITY. 



439 



plete is the reproduction of a lost part. The special- 

 ization of the higher organisms deprives the tissue of 

 the capacity for exact reproduction. As an example 

 of the reduction of this capacity, I cite the reproduc- 

 tion of the tail of lizards, where no vertebrae are repro- 

 duced, but in its place a notochord ; while the squa- 

 mation presents a simpler character than that of the 

 normal tail. The possibility of reproducing the entire 

 organism is restricted, in the multicellular animals, to 

 the germ-plasma, which therefore may be regarded as 

 retaining the characteristic of the protozoon, which 

 reproduces itself by division. But in the multicellular 

 plants the power of reproduction of the entire organ- 

 ism from any of its parts, is retained to a much greater 

 degree than in multicellular animals. The reproduc- 

 tion of plants by cuttings, buds, tubers, and even by 

 single leaves, is well known ; a characteristic which is 

 due to the general distribution of unspecialized proto- 

 plasm throughout the organism. Inheritance of char- 

 acters is in these cases known to be very exact, and 

 there can be here no isolation of the germ-plasma. This 

 isolation is progressively more pronounced as we rise 

 in the scale of specialization of structure, but that it 

 ever becomes absolute, the facts before us forbid us to 

 believe. 



Having thus seen that the plasma of the germ-cells 

 is open to the influence of stimuli, let us see how it is 

 possible that such stimuli can be transmitted to it, and 

 how they could affect growth of the embryo. 



It has been shown that impressions experienced by 

 an animal during one stage of development may be 

 effective in causing the appearance of new structure in 

 a later stage. I have already quoted (Chap. V.) from 

 Poulton the results of experiments on the colors of 



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