230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



found in the developing human embryo. At one stage the pelvic bones 

 are embedded in the body wall some distance below the vertebral col- 

 umn. But in time the iliac elements enlarge, until they reach, and 

 finally embrace, the vertebrae to give rise, finally, to a "sacrum." 



The vertebral column, then, of the Cetacea, must surely be accepted 

 as another instance of "arrested development", and we may take it that 

 it has come about by the gradual decline in viability of the pelvic bones 

 which, as I shall presently show, have become reduced to mere vestiges. 

 In other words the sacral vertebrae do not form "in anticipation" of 

 the contact of the pelvic bones. If the stimulus of the contact of the 

 hip girdle is not given they remain in their "embryonic" state. 



We may fittingly pass now, from "arrested development" to the sub- 

 ject of "vestiges" ; for the one is the forerunner of the other. Wlien 

 any organ tends to be relieved of its functions it begins to decrease in 

 size. But the process, as I shall show, is infinitely slow. Tens of 

 thousands of years must pass before the period of decline ends some- 

 where near extinction. In some cases it may be that the time of the 

 passage is less, but we have no evidence of this, nor can we hope to 

 find any. Hence one cannot but express surprise at the suggestion that 

 has been made more than once, that "nature removes useless organs 

 for the sake of economy." No man could measure the "saving of tis- 

 sue" effected in a single generation, or a dozen generations. Where, 

 then, can be the benefit of such "economy" taking a million years to 

 achieve ? 



The stages by which an organ passes from functional activity to the 

 condition of a vestige are illustrated in a very striking manner, in 

 the case of those lizards known as "skinks." Since the nearly re- 

 lated family Anguidae contains both limbed and limbless forms, and 

 dates from the Lower Eocene, we may take it that the Scincidae are 

 not less ancient. In these skinks one can follow, in four different 

 species of the genus Chalcides^ and one of the genus Lygosoma^ a 

 singularly perfect series in the reduction of both fore and hind legs, 

 showing first a general reduction in size, and then a reduction in the 

 number of the digits from five to two, till finally all that is left is a 

 small, scaly tubercle. Since we know the geological period when the 

 Anguidae started on a precisely similar course we may assume that 

 this process of reduction in the Scincidae began at the same time, 

 some millions of years ago ! 



When we turn to the Sirenia, among the mammals, which also 

 date back to the Eocene, we find the fore limbs in the form of flip- 

 pers like those of a cetacean, and no external trace whatever of the 

 hind limb. But there is a striking series of gradually decreasing 

 vestiges of the pelvic girdle, embedded in the abdominal wall. And 

 here we have a more complete time-scale. The series begins with 



