146 THE FOUNDATIONS OF ZOOLOGY 



As we trace back the ancestral tree, it divides into two branches 

 for the parents, and again into four, and into eight, for the grand- 

 parents and great-grandparents, and so on for a few generations ; 

 but a change soon takes place. 



The student of family records may be permitted to picture 

 genealogy as a tree whose branches become more and more 

 numerous as we go farther and farther backwards from our start- 

 ing-point into the past ; but this cannot be permitted to the 

 zoologist ; for the average number of ancestors in each generation 

 cannot be greater than the average number of individuals in the 

 average sexual environment. It may be very much less, however, 

 for most of the individuals in each generation may fail to perpetu- 

 ate their lines to remote posterity. Now, no animal in a state of 

 nature ranges so far as man in search of a mate ; and the sexual 

 environment of such animals as the fishes in a brook or pond, 

 or the parasites in the intestine of a mammal, is very narrow, as 

 it is in many plants. While new blood no doubt finds its way in 

 from time to time, its influence is more than balanced by the ex- 

 tinction of genetic lines. The series of ancestors of each modern 

 animal is long beyond measure or conception, but the number of 

 ancestors in each remote generation can never be very great, though 

 it may be extremely small. 



The data of systematic zoology also force us to believe the 

 ancestry of all the individuals of a species has been practically 

 identical, except for some slight divergence in the most recent part 

 of their history. 



Instead of picturing the genealogy of a species as a tree, the 

 zoologist must picture it as a slender thread, of very few strands, 

 a little frayed at the near end, but of immeasurable length, and 

 so fine that its thickness is as nothing in comparison. The num- 

 ber of strands is fixed by, but is very much smaller than, the aver- 

 age 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 now as if they had never been. 

 Each of the strands at the near end is important as a possible line 

 of union between the thread of the past and that of the distant 

 future. 



The gist of the whole matter is this : that we must picture 



