4l6 THE GERM-PLASM 



hereditary substance and controlled the body of the cell, varia- 

 tions which had been produced solely by the direct action of 

 external influences on the body of the cell, could no longer be 

 transmitted to the offspring as an inheritance, for they were 

 dependent on the part of the morphoplasm in which they origi- 

 nated, and did not pass into the idioplasm, which comprises 

 all the primary constituents of the species. Every hereditary 

 variation must therefore have originated in the nucleus, even in 

 unicellular organisms — which in this respect therefore resemble 

 the Metazoa and Metaphyta, the sole difference being that in the 

 unicellular form we are concerned with the characters of one 

 cell and not of many. In many unicellular organisms — e.g., the 

 higher Infusoria — the cell-body is very highly differentiated: it 

 may possess a complex arrangement of cilia, undulating mem- 

 branes, trichocysts, and flagella, each having a definite position 

 in the body, — as well as a protective case of a definite form, 

 with a lid capable of being closed ; the animal, moreover, is 

 capable of reconstructing all these parts should any of them 

 become injured. We are therefore forced to admit that this 

 minute organism must have a centre in which the latent germs 

 for all these structures slumber, and from which reproduction 

 may take place. This centre is the nucleus, and modifications 

 of the nuclear matter can alone give rise to transformations of 

 a hereditary nature in the cell-body. 



The fact that species have remained sharply defined as long 

 as we have known them, proves that these transformations occur 

 neither easily nor rapidly. 



The germ-plasm of multicellular forms is obviously also very 

 constant, and the biophors constituting it are capable of nourish- 

 ing themselves and of growing, so as to furnish exact duplicates 

 of themselves when they divide. We could not otherwise under- 

 stand how it would be possible, in spite of the enormous growth 

 of the germ-plasm from one generation to another, for the specific 

 characters, and even the most minute individual characteristics, 

 to be preserved through so many generations. 



The difficulty with which the germ-plasm becomes changed 

 is not so clearly proved by the instances of constancy 

 displayed by some species of ancient Egyptian animals — the 

 ibis and crocodile — through thousands of years, which were 

 formerly advanced by Nageli and myself for this purpose. It 

 may be objected that these species were always subject to the 



