Appendix I. 293 



further; and then the organ will remain as a 'rudiment.' And 

 so it will remain permanently, unless there be some further reason 

 why the still remaining force of heredity should be abolished. 

 This further (or second) reason I found in the consideration that, 

 however enduring we may suppose the force of heredity to be, we 

 cannot suppose that it is actually everlasting ; and, therefore, 

 that we may reasonably attribute the eventual disappearance of 

 rudimentary organs to the eventual failure of heredity itself. In 

 support of this view there is the fact that rudimentary organs, 

 although very persistent, are not everlasting. That they should 

 be very persistent is what we should expect, if the hold which 

 heredity has upon them is great in proportion to the time during 

 which they were originally useful, and thus firmly stamped upon 

 the organization by natural selection causing them to be strongly 

 inherited in the first instance. For example, we might expect 

 that it would be more ditificult finally to eradicate the rudiment of 

 a wing than the rudiment of a feather ; and accordingly we find 

 it a general rule that long-enduring rudiments are rudiments of 

 organs distinctive of the higher taxonomic divisions— i.e. of 

 organs which were longest in building up, and therefore longest 

 sustained in a state of working efiiciency. 



" Thus, upon the whole, my view of the facts of degeneration 

 remains the same as it was when first published in these columns 

 seventeen years ago, and may be summarized as follows. 



" The cessation of selection when working alone (as it probably 

 does during the first centuries of its action upon structures 

 or colours which do not entail any danger to, or perceptible drain 

 upon, the nutritive resources of the organism) cannot cause de- 

 generation below, probably, some lo to 20 per cent. But if from 

 the first the cessation of selection has been assisted by the 

 reversal of selection (on account of the degenerating structure 

 having originally been of a size sufficient to entail a perceptible 

 drain on the nutritive resources of the organism, having now 

 become a source of danger, and so forth), the two principles 

 acting together will continue to reduce the ever-diminishing 

 structure down to the point at which its presence is no longer 

 a perceptible disadvantage to the species. When that point is 

 reached, the reversal of selection will terminate, and the cessation 

 of selection will not then be able of itself to reduce the organ 



