MUTATIONS AND EVOLUTION. 3II 



each living- and extinct representative is characterized by ni;irked 

 degenerative phases. 



The entire argument, that the various series of degenerative 

 stages in the ostrich represent so many orthog'enetic or rectigrade 

 evolutionary series, arising from successional cumulative factorial 

 changes, rests upon the acceptance of their sequential nature in 

 determinate directions. Do the many survivals correspond in 

 a general way with the order of succession of the losses between 

 two extremes, or are they so many haphazard variations which 

 have arisen, apart from any which have gone before, or which 

 will come after? Only by establishing this can we justify their 

 claim to be so many evolutionary series, and to demand the 

 attention of Mendelians as varietal somatic expressions which call 

 for interpretation in factorial terms. 



Among the large number of individual birds, the possibility 

 of securing for each degenerative trend a continuous series of 

 stages between the extremes admits of no question, even though 

 it is held that each has arisen as a discrete factorial change. It 

 has, however, been pointed out by Galton and others that a con- 

 tinuous series of intermediate variations between two extremes 

 does not necessarily represent a germinally successive series, nor 

 indicate an evolutionary trend. Thus Morgan has shown that 

 in DrosophiJa it is possible to arrange a continuous, graded series 

 of eye-colours from one extreme to the other, and also a similar 

 series with perfectly formed wings at one extreme, and no wings 

 at the other. Yet observation shows that the different mutations, 

 however small, appear quite independently one of another, not 

 in any regular succession, and large steps occur as well as small 

 ones. They do net represent a cumulative series, and apparently 

 are not concerned with either progressive or retrogressive 

 evolution of the fruit fly. All grades in human skin colour 

 could be procured from the black of the Negro to the white of 

 the European, yet no one would dream of regarding them as 

 a successional genetic series. 



In a paper, " Genetics versus Palaeontology," Dr. W. K. 

 "Gregory* has fully discussed the evolutionary value of inter- 

 mediate series of fossil stages, contending " that the palaeontologist 

 is dealing with truly successive stages, and not with an arbitrarily 

 selected series of mutants." Where a continuous series of 

 intermediates between two extremes can be shown to be associated 

 with a definite evolutionary trend, the various steps may with 

 good reason be accepted as representing, in a general way, at 

 any rate, the successional stages in the transformation of the 

 particular structure. Thus no one disputes that the pal?eonto- 

 logical series extending from the ancestral pentadactyle foot 

 to the single toe of the modern horse represents, in a general 

 way, the successive retrogressive stages through which the 



* "American Naturalist," vol. 51, Oct., iQi?- 



