PROGRESS IN THE STUDY OF VARIATION. 55 



form, the mean between the two others, or is the inter- 

 vening population found to be practically divisible into two 

 groups of individuals, the one more like the one race, and 

 the other group more like the other race? In any such 

 case there is in fact an opportunity of seeing " swamping ' : 

 by intercrossing, of getting evidence as to whether the two 

 races are capable of freely blending, or whether there is 

 any discontinuity between them. 



In regard to some of the more familiar examples, 

 certain of which are spoken of below, discussion has 

 arisen on the question whether the population occupying 

 the intermediate area where the two races intersrade 

 should be regarded as hybrids between the two, or rather 

 as a still undifferentiated population. A third possibility 

 is that the one race is being or has been directly formed 

 from the other by discontinuous variation, and that in 

 the area of intergradation the process is going on. On 

 consideration, however, it will be seen that, whichever 

 be the true account, the discussion must be a barren 

 one ; for if there is discontinuity between the two forms, 

 whether the two races are newly met or newly dividing 

 the appearances would at any one point of time be the 

 same. This question, like so many others concerning 

 evolution, could only be answered by appeal to observa- 

 tions made at several moments separated by intervals of 

 time. Such evidence is, of course, wanting. Nevertheless, 

 for our purpose a knowledge of the truth of one or other of 

 these views is not immediately needed. The essential fact 

 is that the two forms, though distinct enough to pass for 

 separate species, did they not occupy the same area, on 

 overlapping interbreed in nature. Though in the ordinary 

 sense of the term these forms are not "species," yet they 

 have many of the attributes of species. 



If it is true that the evolution of the one form from the 

 other, or of both from something else, has proceeded by a 

 long series of insensible steps, of which each in turn has 

 been a normal in its own day, surely we should expect to 

 find the intermediate area occupied by an intermediate popu- 

 lation, having an intermediate (if not the mean) for its nor- 



