232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



in the form of the vertebrate skeleton in relation to modes of life, 

 and his inferences thereon are worthy of more serious attention than 

 they have yet received. On this theme of the gradual reduction of 

 the hind limb in the Sirenia, and the Cetacea, he draws attention 

 to the fact that this reduction is to be attributed to the fact that in 

 both groups the body is propelled through the water by horizonal, 

 laterally expanded, tail flukes; while in the Ichthyosauria, wherein 

 the tail was down-turned, and had vertical flukes, the hind limbs are 

 present, though reduced in size. This is really a very important 

 observation. Neither "natural selection" nor "environment" nor 

 "genes" can have had part in these changes, which are due to the 

 mechanical effects of continuous and intensive stimuli on the parts 

 affected, with a corresponding reaction on the adjacent associated 

 structures. 



The wings of birds furnish, perhaps, the most complete history 

 among the vertebrates of the course of the slowing down of the activi- 

 ties of an organ until it ends in complete disappearance. For it is 

 a matter of common knowledge that birds of feeble powers of flight 

 have rounded wings. And there are some species, as for example 

 the wryneck, where the process of shortening sets in after the first 

 juvenile molt. For here the outermost primary is nearly three times 

 as long as it will be in any succeeding molts. Birds on islands tend 

 to become flightless where there is no incentive to flight. The wing is 

 yet complete, but often too small for flight. In the struthious birds 

 we find almost every stage of decadence, from the relatively large 

 wing of rhea — but useless for flight — to the diminutive wings of the 

 apteryx, cassowary, and emu. In the extinct Hesperornis only the 

 upper end of the humerus remained, and in the Dinornithidae the 

 process of reduction proceeded till finally every trace of a wing is 

 lost; even the socket for its articulation has disappeared from the 

 shoulder girdle, itself degenerate in every other aspect. And always, 

 as the wing declines so also does the keel of the breastbone, leaving, 

 in Hesperornis, and the struthious birds, no trace whatever, save in 

 the case of the embryo of apteryx. 



This history of vestiges rules out of court, in the most emphatic 

 way, the contentions that have been made to explain them as the out- 

 come of "natural selection", or of the need for "economy", nor can 

 they be attributed to the effect of the environment. They have fol- 

 lowed a perfectly natural, and orderly sequence, resulting from a 

 continuous sequence of disuse. There are hundreds of cases of ves- 

 tiges, furnished by all kinds of organs, and always and everywhere, 

 they have attained to this condition by the same inevitable process. 



And now let us turn to an exactly opposite trend of evolution, 

 seen in cases of what we call "hypertrophy." Herein organs increase 



