.^08 MUTATIONS AND EVOLUTION. 



increases the chance of a new random mutation along the path 

 already taken." . This is Darwinism of the purest type, but 

 relegatmg the succession of mutations to a mere matter of chance 

 does not take us very far. " Advantage " is freely postulated as 

 a necessity of evolution, but from the considerations to be given 

 below it is sought to show that in the case of the ostrich, at any 

 rate, its assistance is highly problematical. 



During the last twenty years workers on mutation have 

 apparently encountered no experimental type in which successional 

 changes with a definite trend are in progress, so that their 

 scepticism as to its reality can be understood, as also their efiforts 

 to make discontinuous haphazard variations the sole basis of 

 evolution. It may be that few forms are in such a phase at 

 the present time, or that the process is so slow as to be beyond 

 ordinary experimental observation ; but as to its actual occurrence 

 in the past there can be no question.. There may be something 

 in the contention of Prof. M. Caullery* that variability has not 

 been the same in all periods, and also in the view of de Vriesf 

 that the mutative state of a species may differ at different times. 

 In the opinion of the writer, the facts associated with the retro- 

 gressive evolution of the ostrich show that the bird is in a 

 highly mutative state at the present time. The various degenera- 

 tive stages are deemed to be explicable only on the assumption 

 of successional germinal changes with a definite trend ; moreover, 

 as the changes appear to be wholly intrinsic in their origin, 

 and the question of advantage seems to be in no way involved, 

 natural selection is held to have no guiding influence, but only 

 that of final elimination. 



The African ostrich has long been regarded as a degenerate 

 bird, mainly on account of the small size of its wings and the 

 unique reduction of its toes to two. the third and the fourth, 

 of w4iich the latter is already greatly reduced in size compared 

 w^ith the former. Observation shows that losses have taken 

 place in many other directions, particularly as regards the 

 plumage, while the abundance of material available on the ostrich 

 farms of South Africa, along with a large importation of North 

 African birds, reveals that the losses have occurred throughout 

 the continent and proceeded at different rates in different indivi- 

 duals. It is held that the various stages now met with are 

 so much evidence of the manner according to which the losses 

 have occurred in the past, and therefore of the manner in which 

 the germinal changes must have proceeded. 



Briefly, the main facts are as follows. In most ostriches the 

 under surface of the wing is naked except for the presence of 

 a single incomplete row of under-coverts. Survivals of a second 

 and even of a third row occur very rarely, and all numerical 



* " Science," vol. XLIII, April 21, 1916, p. 555. 



t " Species and Varieties : Their Origin by Mutation," London, TQ05. 

 P. 699. 



