August 20, 1920] 



SCIENCE 



167 



germ plasm controlling the plumage and the 

 structural details of the wings and legs re- 

 main unchanged, and it is in these that we 

 have similar degenerative changes in progress. 

 In studying the small differences which dis- 

 tinguish species we may overlook the main 

 essentials in which they agree. 



The individuality and separability of the 

 germ factors in the ostrich are well exempli- 

 fied in the fact that while differential changes 

 have taken place in some respects common 

 changes have taken and are taking- place in 

 other respects. We may respectively designate 

 as parallel the mutation found in the two spe- 

 cies of Drosophila and the numerous ones met 

 with in the two species of Struthio, because 

 they occur in different species, but it must be 

 with the understanding that the changes pro- 

 ceed in similar parts of the germ plasm which 

 are common to both, and would go on were 

 there no specific differences. 



AVhether parallel mutations ever occur in 

 germ plasms not genetically related may well 

 be doubted. It may be that similarity of 

 changes always implies similarity of origin in 

 corresponding parts of the germ plasm. There 

 is abundant evidence that the fourth, outer toe 

 of the ostrich is well on the way towards dis- 

 appearance, and when this is effected we shall 

 have only the single middle toe in two such 

 widely separated animals as the ostrich and 

 the horse. While to institute a close com- 

 parison of the limbs of the two animals seems 

 to strike at the roots of our morphological 

 ideas it may well be that the degeneration in 

 the toes of Struthio is an expression of 

 changes in its germ plasm of a fairly similar 

 nature to those which have been effected in 

 the germ plasm of Equus, and is referable to 

 separable parts of the germ plasm which tie 

 two have or had in common. All that this 

 involves however is the recognition of the fact 

 that the fundamental parts of the limbs of 

 digitate vertebrates take their origin from 

 corresponding parts of the germ plasm com- 

 mon to all. Osborn^ must have had occur- 

 rences of this kind in mind when he wrote: 

 3 Amer. Nat., April, 1915. 



" Similar rectigradations may arise in all the 

 descendents of similar ancestors at different 

 periods of time ; they always give rise to paral- 

 lelism or convergence between the members 

 of related phyla." To go beyond digitate 

 limbs, we may presume that the characteristics 

 of the chordate phylum — dorsal tubular nerv- 

 ous system, notochord, visceral arches and 

 clefts — arise from separable parts of the germ 

 plasm which are common to all the members 

 of the phylum, and distinct from other parts 

 which give rise to the characters serving to 

 distinguish the classes of the chordates. 



In whatever way we conceive the germinal 

 changes to be going on in the two-toed ostrich 

 it is clear they are not limited to the African 

 genus Struthio among the flightless birds. 

 For we have degenerative changes of a fairly 

 similar nature represented in the three-toed 

 American ostrich {Rhea) and in the wing of 

 the ISTew Zealand Apteryx, while the Moas had 

 apparently lost their wings altogether before 

 becoming extinct. "Where exactly similar, 

 these may be termed parallel mutations, but 

 it seems more in harmony with modern 

 genetical ideas to think of them as taking 

 place in separate parts of the germ plasm 

 which the sub-class has in common. In the 

 Ratit« as a whole we seem to have one of 

 those groups of animals, so often represented 

 in the evolutionary series, in which high spe- 

 cialization in certain directions is accom- 

 panied by marked decadence in other respects 

 and which in the end result in extinction. 

 When they are alike the changes may be de- 

 scribed as parallel mutations, but it is sought 

 to emphasize that they take place in a similar 

 manner in those parts of the germ plasm 

 which the group has genetically in common. 



Though the living representatives are so 

 widely separated, the germ plasm controlling 

 the plumage, wings and legs of the Eatitffi as 

 a whole is manifestly subject to some common 

 intrinsic influence which expresses itself in 

 the gradual loss of these structures, the proc- 

 ess proceeding more rapidly in some than in 

 others and sometimes modified in different 

 ways. Many of the facts of degeneration pre- 

 sented by the ostrich incline one to attribute 



