516 General Notes. [May, 
very similar to those published by Professor Gill’ in 1882, who 
regards these characteristic structures as having been derived 
_ from greatly hypertrophied integuments of hind-limbs analogous 
to such as are developed, for instance, to the hind-limbs of the 
eared seals, while the osseous elements have been inversely atro- 
nected with the organs of generation. It may, I think, be re- 
garded as a fact that there is no evidence to show that develop- 
ment does not attempt to recapitulate in a disguised form, in the 
cetacean fœtus, the outgrowth of the hind-limb as seen in a 
normal mammalian embryo. 
In the mammal the pectoral limb is the first to appear, the 
pelvic appearing last. If the flukes be regarded as the outward 
vestiges of hind-limbs or pedes, then will the embryos of ceta- 
ceans and sirenians conform to this law presiding over the order 
of appearance of the limbs, which, so far as I am aware, is regnant 
without exception within the limits of the vertebrate class. The 
dorsal fin with which the flukes have so often been mistakenly 
compared, is not present in all cetaceans; is absent in all sirenians; 
is not constant in position in different genera of the former; is 
sometimes a mere carina, dorsal ridge or hump; its vascular an 
nerve supply is different from that of the flukes; it develops after 
the latter, showing that it is a later acquirement ; it has absolutely 
no connection with muscles directly or indirectly by tendons as 
have the flukes, so that I regard the comparison of the dorsal fin, 
which is a mere dermal fold, with the flukes, as expressing a mis- 
taken apprehension of anatomical homologies, and not justified in 
the face of the fact that the flukes are never absent, and always 
appear laterally or serially in the position of a hind limb-fold, _ 
though backwardly displaced. It must, however, be stated that I 
distinctly disavow the affirmation that the flukes are homologous 
with more than the pedes of normal forms. The older views 
which intimated that the flukes were the representatives of limbs 
or of feet, it is not worth while to discuss, as these writers did not 
possess the data upon which to base any reasons for their 
opinions, which seem to have been in the main intuitional. 
The hypothesis which is offered to account for the flukes as the 
distal vestiges of limbs rests upon the following arguments :—1. 
The mode of outgrowth of the flukes in the embryo, prior to the 
dorsal fin, at the end of the sides of the tail, at first as a pair of 
low rounded lobes or folds of skin, containing mesoblast, which 
become gradually falcate, and which expand posteriorly so as to 
leave a notch over the end of the tail between their inner edges. 
The hind margin of the flukes answering to the terminal border 
_ of the pes or the ends of the suppressed toes or the integuments 
extending beyond them, and their anterior margin to the outer 
_ n Scientific and popular views of nature contrasted. A lecture delivered in the 
National Museum, March 11th, 1882. Pp. 10-11. Washington, Judd & Detweiler. 
