Febki'ARY 1, 1895.] 



SCIENCE. 



123 



relations ai-e concerned, although at the end 

 of the period the inluibitants would cer- 

 tainly not be descended from the Baltimor- 

 ians of our day, but from only a very few 

 of them. Most of our lines would be ex- 

 tinct, and the few which survived would 

 include most of the Baltimorians of the year 

 2900 among their descendants, who, while 

 unconscious of their common origin, would 

 be allied with each other by common de- 

 scent fi'om their virile and prolific ance,stors 

 of the year 1894. 



This is proved indirectly but conclusively 

 by genealogical statistics, and while a thous- 

 and years are hut as yesterday in the his- 

 tory of species, zoological considerations 

 furnish e^-idence that allied animals at two 

 successive geological periods must be re- 

 lated like these successive generations of 

 Baltimorians. Of all the individuals of a 

 species which lived at a given period, very 

 few would have descendants at a later per- 

 iod, and these few would be the common 

 ancestors of all the individuals which repre- 

 sent the stock at the later period. 



The extinction of species is a familiar 

 conception. The extinction of the lines of 

 descent from individuals is no less real, 

 and infinitely more significant in the study 

 of inheritance. 



As we trace back the ancestral tree it 

 divides into two branches for the parents, 

 and again into four and eight for the grand- 

 parents and great-grandparents, and so on 

 for a few generati(ms, but a change soon 

 takes place. The student of family records 

 may be jHTniitted to pictui-e genealogy as a 

 tree whose branches become more and more 

 numerous as we get faither and farther 

 from the starting point; but this cannot be 

 permitted to the zoologist. 



On the contrary, we must acbnit that, on 

 the average, the number of ancestors in each 

 generatiim can never be gi-eater than the 

 number of individuals in the average sexual 

 environment. It may be very nmch less, 



however, since most of the individuals in 

 each generation must fail to perpetuate 

 their lines to remote descendants. 



Now no animal in a state of nature 

 ranges so far as man in search of a mate, 

 and the sexual environment of many plants 

 and animals, such as the fishes in a brook 

 or a pond, or the parasites in the intestine 

 of a mammal, is very narrow. While new 

 blood, no doubt, finds its way in from time 

 to time, this is more than balanced by the 

 extinction of genetic lines. The .series of an- 

 cestors of each modern organism is long be- 

 yond measure, but the number of ancestors 

 in each remote generation can never be very 

 gi-eat, though it may be extremely small. 



The data of systematic zoology also force 

 us to believe that tlie ancestry of all the 

 individuals of a species has been identical, 

 except for the slight divergence in the most 

 recent part of their history. 



The zoologist must picture the genealogy 

 of a species not as a tree, but as a slender 

 thread, of verj- few strands, a little frayed 

 out at the near end, but of immeasurable 

 length and so fine that the thickness is as 

 nothing in comparison. The number of 

 strands is fixed by. but is much smaller than, 

 the average sexual environment. If we 

 choose we may picture a fringe of loose ends 

 all along the thread to represent the ancient 

 animals which, having no descendants, are 

 to us as if they had never been. Each of 

 the strands at the near end is important, as 

 a possible line of union l)etween the thread 

 of tlie past and that of the distant future. 



The gist of the whole matter is this, that 

 we miist picture this slender tliread as com- 

 mon to all the individuals of the species, 

 whose divergence from each other is infini- 

 tesimal compared wilh the ancestry they 

 share in common. 



The l)ranches of a human genealogical 

 tree diverge for a few generations bj- geo- 

 metrical progression, but we soon find traces 

 of a change, and if the record were long 



