V/EIVS OF G ALTON AND WEISMANN ON INHERITANCE 1 47 



this slender thread as common to all the individuals of the species, 

 whose divergence from each other is infinitesimal as compared 

 with the ancestry which they share in common. The branches of 

 a human genealogical tree diverge for a few generations by geo- 

 metrical progression, but we soon find traces of a change, and if 

 the record were long enough to have any zoological significance, 

 we should surely find all the members of the species descended 

 from a few ancestors in each remote generation, and these few 

 the common ancestors of all. So too of the common ancestors of 

 divergent species, or those of larger groups; if one metazoon is 

 descended from pre-Cambrian unicellular ancestors, the same uni- 

 cellular individuals must have been the common ancestors of all 

 the metazoa ; and we may be confident that there were not very 

 many of them in any one generation. It is quite possible that 

 they were so few as a single pair, or even one. 



There is nothing novel in all this. Galton has himself 

 devoted an appendix to the mathematical study of the extinction 

 of family names ; although he, like other writers on inheritance, 

 seems to forget it when he assumes that the remote ancestors of 

 two persons were, like the parents, distinct individuals, and that 

 the child must therefore have twice as much ancestry as either 

 parent, and consequently twice as much variety, unless there is 

 some way to cancel half of it at each step. 



I called attention to the bearing of this convergence of ances- 

 try on the problem of inheritance, in 1883, in words which still 

 seem clear ; although the views of both Galton and Weismann on 

 variation are based on the unfounded assumption that each 

 sexual act brings together two totally dissimilar sets of factors, 

 instead of two factors which are alike in innumerable features, 

 for each one in which they differ. 



My statement is as follows: "In order to breed together, 

 animals must be closely related ; they must belong to the same 

 species, or to two closely related species. Since the individuals 

 which belong to two closely related species are the descendants 

 of a common and not very remote ancestral species, it is clear 

 that almost the whole of their history has been shared by them 

 in common ; all their generic characteristics being inherited from 

 this ancestor. Only the slight differences in minor points which 



