THE MASKING OF RETROGRESSION 117 



are progressions and others retrogressions. If we think of each of 

 the component variations separately, and, if necessary, analyse 

 them, we see that every progression must be built on the develop- 

 ment as presented by the parent, must consist of an addition 

 after a complete recapitulation (as, far as that character is concerned) 

 of the life-history as presented by the parent ; whereas every 

 retrogression must consist of a subtraction from the complete 

 recapitulation a reversion. Think how we will, puzzle how we 

 may, we must always arrive at the conclusion that retrogression is 

 identical with reversion. 



192. There are, then, two kinds of 'reversion.' (i) The 

 individual may revert ancestor-wards through retrogression 

 through the total loss of a developmental potentiality from the 

 germ-plasm. (2) He may reproduce a dormant ancestral trait. The 

 latter, in appearence at least, is also reversion, even though nothing 

 is lost to the germ-plasm even though the more modern trait 

 becomes in its turn merely dormant. Most biologists limit the 

 term reversion to the more or less glaring reproduction of a latent 

 ancestral trait, but only, I think, because they have not as yet 

 thought of retrogression in connection with reversion. 



193. The reason why retrogression seldom* presents the appear- 

 ance of reversion, and why, therefore, there has been a general 

 failure to recognize the relations between the two, arises from the 

 fact that, since the parts of the individual are independently 

 variable, retrogressive and progressive variations are usually closely 

 intermingled, with the result that the nature of the former is masked. 

 They cannot be clearly discerned as reversions. Moreover, in old- 

 established types especially, the majority of variations tend to be 

 of very small magnitude. Sometimes, however, retrogression is 

 extensive and affects some conspicuous character or a number of 

 characters together, and then, as when a prize breed loses its 

 special traits, we are able to recognize its nature. In by far 

 the great majority of instances, however, retrogression is not 

 obviously identical with reversion. Not sight, but thought, con- 

 vinces us of the identity. Thus, while the wing of a bird, whose 

 latest descendants have lost the power of flight, was evolving from 

 the reptilian fore-limb, much was lost as well as gained. Conse- 

 quently when the wing became useless, for example in the apteryx, 

 and retrogression en masse set in, there was no return to the 

 reptilian limb, for the major part of the latter had already under- 

 gone retrogression during its change to an organ of flight. 



194. Every individual varies from his parent in an almost 



