312 MUTATIONS AND EVOLUTION. 



evolution of the fcot of the horse has taken place, and that 

 corresponding successional changes must have been undergone by 

 the germ plasm. The series revealing the increase in size and 

 complexity of the teeth, so clearly show^n in Gregory's paper, 

 would presumably be acceptable to all as a successional cumulative 

 series, revealing the orthogenic nature cf the evolution of the 

 teeth of the modern horse, and also calling for germinal changes 

 of a corresponding nature. 



The claim for the successional nature of the different 

 retrogressive series in the ostrich is even more insistent, from 

 the great number of individual northern and southern birds 

 available for observation along with complete embrvonic stages. 

 In every case an evolutionary retrogressive trend is indicated 

 when comparison is made with other birds. The gradual ordinal 

 loss from the rows of under- and upper-co^•erts and remiges. 

 each plume in piecemeal stages ; the slow disappearance of the 

 down, each member also in piecemeal fashion ; the continued 

 stages in the reduction of certain of the digits of the wing, and 

 especially of the foot, all indicate a definite determinate trend, 

 and can only be the somatic manifestation of continued, deter- 

 minate factorial changes in the germ plasm of the ostrich. Each 

 germinal loss or alteration is held to be discrete, and each renders 

 the next one in the series possible ; nothing fortuitous or irregular 

 in the order in which they proceed has been encountered. 



The degenerative changes are cf the same character through- 

 out the race, for corresponding stages are found in both northern 

 and southern birds, and may, therefore, be presumed to be the 

 same throughout the continent. Individual differences occur, 

 indicating the diff'erent rate at which the changes take place, 

 affording the data as to the manner in which they are proceeding, 

 but nowhere is there any suggestion of a divergent trend ; the 

 direction of each series is constant throughout. It is this 

 continental uniformity of the losses which is so strongly 

 suggestive of the intrinsic nature of the germinal changes involved, 

 and of the absence of any adaptive consideration. The germ 

 plasm throughout the race must have undergone, and be still 

 undergoing similar degradation changes, and though of their true 

 nature we know nothing, they can only be conceived as regular 

 and methodical. 



The losses in the ostrich are proceeding with such regularity 

 and in such definite directions that there need be no hesitation 

 in predicting their course in the future. Thus the single row 

 of under-coverts may well be expected wholly to disappear, 

 following upon the loss already of at least fourteen of its 

 members, and that of the second and third rows. The upper- 

 coverts are greatly reduced in some birds, and, along with the 

 remiges, may be expected to follow the way of the under- 

 coverts. There can be no question that the already reduced 

 fourth toe will, in process of time, leave the ostrich with only 



