PROBLEM OF ORIGIN OF SPECIES 55 



direction, shortening the checkers and transforming the rows suc- 

 cessively into narrow bands, eventually reaching the eleventh row, 

 where we find only one or two complete steps, followed by a graded 

 series of 4 to 6 steps, less and less decided, until we lose every trace 

 of them. So finely graded are these steps in some females that it is 

 difficult to locate the vanishing-point. 



Unless the process of transformation is arrested by the extinction 

 of the species, or through the intervention of some more potent 

 modifying influences than have thus far appeared, the fate of both 

 posterior bars is irrevocably sealed. Granting that natural selection 

 may be credited with strengthening the iridescent splendor of these 

 bars, I believe that the orthogenetic influences are bound to prevail 

 here as in the white-breasted species. 



But is there any direct proof that the transformation is actually 

 making progress to-day? May not these transitional steps go on ap- 

 pearing generation after generation, without ever making any per- 

 manent progress? 



We have to concede that we cannot follow the processes that 

 reveal themselves in steps. We can at most only see what is done 

 not the doing. We are entirely in the dark as to the time required 

 to carry the change through a single row of feathers. But we know 

 that this has been done in three other species of the same family. 

 We see that after it is done, not before, the transitional steps appear in 

 the next and last row. Moreover, and this is as close as we can 

 hope to get to actual seeing, we find that progress of just the kind 

 we are looking for is certainly made in passing from the Juvenal to the 

 adult plumage. This is an ontogenetic change of a few weeks, which 

 we can easily demonstrate by experiment to be progressive and con- 

 tinuous. The corresponding phylogenetic advance has left no other 

 record, and hence we only know that it took time that it was not 

 a momentary salt. In the adult plumage, one or two full steps are taken 

 beyond the Juvenal stage, and taken precisely at the points premarked 

 by transitional steps. The number of transitional steps is increased 

 at the same time. 1 



As the next and last illustration, I take a case in which the bars 

 are verging to complete obliteration. The well-known w r ild stock 

 dove (C. anas) of Europe may serve as a convenient and instruct- 



1 One point here should not escape attention; namely, that the transitional 

 steps in Ocyphaps form a linear series; but there is nothing artificial or arbitrary 

 about it. It is a small-number series, each element of which stands in an appointed 

 place, and marks the height to which the transformation process rose at that point 

 in its course. Such a series cannot be open to the objections which De Vries has 

 very justly made against large-number series, the elements of which are collected 

 at random and then arranged arbitrarily to display transitional continuity. 



In the Ocyphaps series there is some fluctuation, the series varying in length, 

 but always advancing in one predetermined direction, like a tidal flow guided 

 along a prepared channel, and flowing to varying distances, according to the 

 initial momentum. 



