MUTATIONS AND KVOLUTION. 313 



the single middle toe. and, althougli this may for the time lieing 

 incidentally result in some mechanical advantage to the bird 

 in running, evidence is afforded that the retrogressive force will 

 not stop here. Could we conceive of the ostrich surviving long 

 enough, there need be no doubt that it would ultimately suffer 

 the loss of all its toes as well as its plumage. In the end natural 

 selection is powerless against the degenerative influences at work 

 in or on the germ plasm ; it cannot stay them indefinitely, nor 

 can it direct them ; it can only eliminate when the stages have 

 proceeded so far as seriously to interfere with the welfare of 

 the bird. The ostrich appears to be a forcible illustration of 

 the remark by Dr. M. M. Metcalf* : " Natural selection has con- 

 sisted largely in the elimination of species whose unfitness was 

 shown only after a long period of indifferent orthogenic develop- 

 ment." 



Elimination by natural selection will not be simultaneous for 

 the race, but will be largely determined by the individual rate of 

 retrogression. It will at first concern those particular individuals 

 in which retrogression had proceeded furthest, but in time the 

 extinction stage will be reached by others, and ultimately by the 

 entire race. A course of degeneration entered upon need not, 

 however, necessarily be completed before a halt is made, as is 

 here assumed. Thus the modern horse, which ancestrally has 

 lost four of its digits, seems to afford every proof that retro- 

 gression in this particular direction has now ceased, and that no 

 fear need be entertained that it will ultimately lose its remaining 

 middle toe. as would be the case if degeneration proceeded 

 consistently. The vestigial survivals of the hind limbs and 

 girdles in the boas and pythons among snakes, and also among 

 cetaceans and sirenians, may either indicate a cessation of the 

 original degenerative influence, or they may represent instances 

 in which retrogression has proceeded more slowly than in related 

 forms, and in which further losses will yet take place. Similarly 

 with the human ear and scalp muscles and the vermiform 

 appendix: their persistence in varying degrees may either 

 indicate a cessation of degeneration, or the last survivals of 

 structures the loss of which will yet be completed. 



That the degenerative stages presented by the ostrich at the 

 present day justify us in making predictions of the above nature, 

 as well as others which could be mentioned, should suffice to 

 establish the claim that the losses are truly orthogenetic or 

 rectigrade. They have been real in the past for ages and ages, 

 and no re^ison whatever exists for thinking the same trends will 

 not be maintained in the future. Instead of being satisfied with 

 casual, seemingly fortuitous factorial changes, the ]\Iendelian 

 has to face the bigger issues of continued, cumulative changes in 

 many directions, common to the germ plasm of a whole race. 



* .'• Journ. of Heredity/' vol. VII., Aug., 1916, p. 357. 



