EVOLUTION— PYCR AFT 231 



Eotherium of the Middle Eocene, some 20,000,000 years ago. Here 

 Ave have a complete innominate bone, but reduced in size. In Eosiren, 

 of the Upper Eocene, the obturator foramen had disappeared, and 

 the acetabulum greatly reduced : Halifherium^ of the Middle Oligo- 

 cene, shows a still further reduction of the pubic and ischial ele- 

 ments. In Metaxitherium^ of the Middle Eocene, the pelvis is re- 

 duced to a mere protuberance, and the acetabulum is closing up. 

 Finally, in the dugong {Halicore) and manatee {Manatus) of today, 

 nothing is left of the pubis, and but the merest trace of the acetabu- 

 lum, while the innominate, as a whole, has become a mere rod, as 

 shown on plate 5. 



These changes, be it noted, were accompanied by other changes ex- 

 ternal and internal, sufficiently great to divide the members of this 

 continuous series into distinct genera, whether we allot 15,000,000 

 or 20,000,000 years to this chain of evolution does not greatly matter ; 

 we have geological evidence that the process was infinitely slow. 

 How, then, can we hope to "prove" or refute, the "transmission of 

 acquired characters" by experiments carried on with white mice, for 

 a dozen generations or so. 



Let us turn now to a precisely similar story in regard to the hip 

 girdle of the Cetacea, though here we are confined to the evidence 

 afforded only by species still living. But it brings to light another 

 aspect, the significance of which seems to have been overlooked, 

 and this concerns the fact that these retrogressive changes vary 

 somewhat conspicuously in different individuals. The reduction is 

 least in the Greenland whale. But even here the pubis is barely dis- 

 tinguishable, and the acetabulum a mere depression. The pelvic 

 limb is represented by the femur, and a portion of the proximal end 

 of the tibia reduced to a mere nodule of cartilage. Both in size and 

 shape the femur presents wide differences, ranging from a rod-shape, 

 to a quadrangular plate of bone, and the tibial cartilage is no less 

 variable. And the same is true of the fin whale {Balaenoptera phy- 

 salus). The innominate is little more than a mere rod, but with a 

 bifurcated posterior extremity representing the last traces of ischium 

 and pubis. There may be a roughly rod-shaped femur, with a 

 nodule of cartilage answering to the proximal end of the tibia, or 

 there may be no more than a small nodule of bone representing the 

 femur. The acetabulum has vanished. In the Odontoceti the process 

 of degeneration has proceeded still farther. Only in the sperm whale 

 Physeter is there ever as much as a nodule of cartilage answering to 

 the femur. That the increments due to use are more rapid than 

 the processes of absorption is a matter w^hich cannot be determined. 



My friend Prof. O. Abel in his fine book on "Palaeobiologie", has 

 brought together a remarkable collection of facts in regard to changes 



