EVOLUTION— PYCRAFT 229 



the Sirenia (manatees), they were entirely aquatic, and in like man- 

 ner, in consequence, assumed the same general form in regard to the 

 body as a whole, and the flipper in particular. But when we come to 

 examine the skeleton of this limb we find more surprising differences. 

 Here cartilage, in the adult stage, had no place. Moreover the fore- 

 arm is found to have become so shortened as to require expert exami- 

 nation to distinguish its component elements — radius and ulna — from 

 the carpal, or wristbones, and these, in turn, often by no means easy 

 to distinguish from the phalanges, are crowded together to form a 

 close mosaic. In the "finner whale", it may be remembered, one digit 

 had apparently been "squeezed out" so that only its terminal portion 

 remained. We find the same "squeezing" in regard to one of the 

 digits — the second — in some species of Ichthyosaur's paddles. But 

 here, only the extreme proximal end is wanting. A further peculiarity 

 is found in a row, or sometimes two rows, of ossicles along the post- 

 axial border of the hand. These were nodules of bone formed in an 

 external "fin-membrane", apparently, as one might say "to give in- 

 creased width", though this phrase must not be taken to imply "a 

 means to attain an end." We shall be nearer the truth in assuming 

 some peculiarity in the mode of swimming, bringing into play a 

 marked stimulus to the skin along this border of the flipper. This 

 may well have been the case, for the tail of the Ichthyosauri was 

 directed downward, and carried vertical "flukes." 



Here, then, in all these cases, we have evolution and decadence going 

 on at the same time, a fact well worth bearing in mind, for it has not 

 yet received the attention it deserves. 



And now let me pass to consider the theme of "arrested develop- 

 ment", to which I have already referred. Its more important aspects 

 can be briefly reviewed. We can find no clearer, or more convincing 

 examples of this aspect of evolution than are furnished by the Cetacea, 

 and, in the first place, by the flippers of the rorquals, including the 

 Humpback. For in these the ossification of the phalanges is never 

 completed, each end being capped by masses of cartilage. In the em- 

 bryonic development of the manus of all the land animals there is a 

 stage which exactly corresponds to this permanent, adult condition, 

 found in all but the most primitive cetaceans. More striking still is 

 the vertebral column. In all the higher vertebrates the lumbar verte- 

 brae are free, and succeeded by a series of fused bones which have 

 become specially modified by having become immobile owing to the 

 grip of the pelvic bones. They are known as the "sacral vertebrae." 

 In the Cetacea there is no sign of sacrals. Tlie vertebrae pass insensi- 

 bly from the lumbar to the caudal series. But there is a precisely 

 similar absence of "sacrals" in the developing vertebral column of all 

 the higher vertebrates. For the sake of contrast I cite the condition 



