334 THE AMERICAN NATURALIST [Vol. LIII 



due to some wholly intrinsic cause, and seeing that it 

 ai¥ects all the members of the race and has been operative 

 for a long period we may conclude that it is transmissible 

 and acts continuously from generation to generation. 

 The many stages represented also give some justification 

 for supposing that whatever the cause of the factorial 

 changes may be it varies in intensity in different mem- 

 bers of the race, being less active in individuals where the 

 loss of plumes is small as compared with others in which 

 the loss is greater. For example, the causative agent 

 bringing about the loss of the plume factors must be less 

 in intensity or less active in 42-plumed ostriches than in 

 33-plumed birds. We may with good reason expect that 

 the selection for breeding of the high numbered birds 

 will arrest the rate of degeneration of the race in this 

 particular feature, while on the other hand the selection 

 of the low-numbered birds will tend to accelerate the rate 

 at which the factorial losses are taking place. Where 

 therefore the germ plasm of a race is in a continuously 

 changing phase, as in the ostrich, we can hope to retard or 

 accelerate the changes by selecting individuals differing 

 in the degree to which they are under the influence of the 

 causative agent. It is submitted that in this sense we can 

 say that ''a purely external agent, the continued selec- 

 tion of personal somatic qualities, will alter the germ 

 plasm." 



We can not hope that the continued selection of 42- 

 plumed birds will in the end give to the farmer ostriches 

 with a still higher number of remiges, as the factors for 

 the plumes beyond these have in all probability disap- 

 peared from the race, and there is no evidence that the 

 cause of the factorial changes is effective in a plus but 

 only in a minus direction. On the other hand the con- 

 tinued selection of 33-plumed birds may reasonably be ex- 

 pected to accelerate the loss of the remiges, by leading to 

 a more -rapid loss of the factors. Owing to the present 

 degenerative forces at work in the ostrich we can by 

 selection hope to modify the germ plasm in a minus direc- 



