VJEIVS OF G ALTON AND WEISMANN ON INHERITANCE 1 45 



are not 452 distinct persons, but only 149; many of them, in the 

 more remote generations, being common ancestors of all three in 

 many lines. If the lines of descent from the unrecorded ancestors 

 were interrelated in the same way, as they would surely be in an 

 old and stable community, the total ancestry of these three per- 

 sons, for seven and a half generations, would be 378 persons in- 

 stead of 1 146. 



Few of us know even the names of all the living descendants 

 of each of our sixty-four ancestors of the sixth generation ; and, so 

 far as our own choice is concerned, marriage with one of them 

 may be an accident ; for the probability of such a marriage de- 

 pends upon things which are in great part independent of us, 

 upon the size of the circle of acquaintances, and the distance 

 of the places to which ancestors wandered. For if each pair of 

 ancestors had only four ' children, more than twelve thousand of 

 their descendants may now be living (4048 + 8096). 



If a city like Baltimore, where the strangers to each one of us 

 outnumber our acquaintances a thousand fold, could be quaran- 

 tined against people from outside for a thousand years, each suc- 

 cessive generation would be much like the present, so far as known 

 relationships are concerned, although, at the end of this period, 

 the inhabitants would not be descended from the Baltimoreans of 

 our day, but from only a very few of them. Most of our lines 

 would be extinct; and the few that survived would include most 

 of the Baltimoreans of the year 2898. 



All this is proved, indirectly but conclusively, by genealogical 

 statistics ; and while a thousand years are but as yesterday in the 

 history of species, zoological phenomena furnish evidence that 

 allied animals must be related to each other, at two widely sepa- 

 rated generations, like these successive generations of Baltimoreans. 



Of all the individual animals which make up the species at a 

 given period, very few will have descendants at a later period, and 

 these few will be the common ancestors of all the individuals 

 which represent the stock at the later period. 



The extinction of species is a familiar conception. The extinc- 

 tion of the lines of descent from individuals is no less real, and, 

 in the study of inheritance, vastly more important ; for it is the 

 fact of which the extinction of species is only an expression. 



