80 CcU-Pcdigvces 



already mentioned in the first Part. We need therefore 

 not deal with that question in -this Section. 



Not the organisms, but the cells, are therefore the 

 units of the theory of heredity. One has to go back to 

 these for a clear understanding. In the practical pedi- 

 grees of the animal- and plant-breeders of course only 

 the individuals figure, but for a scientific insight, these are 

 not sufficient, as is well known to the greatest authorities 

 among breeders. 



Here the germ-cells {tgg- and sperm-cells) come into 

 the foreground for consideration. They are the material 

 parts of the parents from which the children issue, and 

 hence form the material bond between the successive 

 generations. For every germ-cell we may trace a series 

 of ancestral cells back to the last preceding generations. 

 In this way we may proceed further, and follow up the 

 pedigree of the germ-cells through a series of generations. 

 The great scientific significance of these sequences of cells 

 has been strongly emphasized by Weismann ; they form, 

 without doubt, the basis for the theory of cell-pedigrees. 



But this kind of treatment leads to a one-sided con- 

 ception of the problem. We ought rather to trace the 

 ancestral line of all the cells of the entire body back 

 to the first cell from which the organism started. It is 

 true that thereby the task becomes much more extensive 

 and complicated, and it is a question whether a sufficient 

 anatomical and ontogenetic basis is at hand for its solu- 

 tion. Nevertheless it is only in this way that we can 

 approach a uniform treatment of the subject, and group 

 the available facts in such a way that they do not de- 

 ceive us, nor lead us to an overestimation of the signifi- 

 cance of isolated cell-sequences selected by us arbitrarily. 



We should, therefore, trace out the pedigree of the 



