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fact of organic repetition, and have no more reason in the one 

 case than in the other to view heredity as the function of any 

 special organ. We may define heredity as the property of 

 organisms with as much propriety as the chemist treats crystal- 

 lization as a property of sugar. The cells know, as it were, 

 how to arrange themselves repeatedly into similar colonies or 

 compound individuals, just as the molecules of a chemical com- 

 pound take repeatedly the same crystal form. 



The causes of crystallization and of heredity are equally 

 unknown ; we can merely expect for the future that to which 

 the past has accustomed us. We have no better reasons for 

 expecting to find that the adult is definitely prefigured in the 

 germ-cell that we have for supposing that the crystallographic 

 forms or other properties of inorganic materials can be deter- 

 mined by microscopical examinations of the substances in solu- 

 tions or in amorphous states. The germ-cells with their chro- 

 mosomes and other internal organs do indeed carry the organic 

 sequence from one generation to another, but this fact gives us 

 no warrant that they contain any parts or particles which will 

 afford a general explanation of evolution. And even if the 

 germ-cells do contain some feature of special bearing upon 

 heredity, it does not alter the probability that the results of the 

 agencies operating in the germ-cells are shown to best advantage 

 in the completed organisms. Sperms and egg-cells are them- 

 selves organisms, quite as truly as the elephants and whales, 

 but their infinitesimal size, which kept them unknown and mys- 

 terious so long, does not warrant us in ascribing to them any 

 gratuitous mysteries, nor in failing to appreciate that evolution 

 is a motion of the specific network of descent. 



Whatever the nature and functions of nuclear organs may be 

 in different groups of animals and plants, we may expect that 

 these organs and functions will find their primary explanation 

 and relations in the evolutionary network of descent, rather than 

 as affording an independent basis for theories of heredity. 

 Neither the relations of individual organisms to environment, 

 nor the possibility that germ-cells have predetermining relations 

 to adults, will justify us in leaving out of account the network 

 of descent in which the evolution of species goes forward. 



