1885. ] Embryology. 515 
instances, each jump was made in a different direction from the 
last, so that it was a matter of some difficulty to pursue the little 
creature! I was exceedingly interested in this first capture— 
never having seen or heard of one before. One of our farm hands 
told me that it was a “kangaroo mouse.” After that I saw one 
occasionally, and my recollection is very clear that Godman does 
not overstate its ability to jump. I have never seen or heard of 
one in this region.— Charles Aldrich, Webster City, Iowa, March 
5, 1885. 
EMBRYOLOGY .! 
ON THE PROBABLE ORIGIN, HOMOLOGIES AND DEVELOPMENT OF 
THE FLUKES OF CETACEANS AND SIRENIANS.—We have seen that 
the development of the Physoclist fishes (Am. NaTJRALIsT, 1885, 
pp. 315-317), shows that the translocation of the pelvic fins for- 
ward is accomplished in some forms in about twenty-four to forty- 
eight hours, to a position more or less in advance of the pectoral, 
If a limb-fold can be translocated forwards in a vertebrate embryo 
from its archaic site, there is no reason to doubt that under cer- 
tain conditions it might be translocated in the other direction or 
backwards. A process of translocation of the distal end of the 
pelvic limbs seems to have occurred in the cetaceans, as a conse- 
quence of which the pes has acquired a new position far to the 
rear of that which it occupies in normal mammals, and this seems 
to have been accompanied by processes of atrophy in some direc- 
tions and hypertrophy in others. 
_ The researches of Struthers, Flower, Reinhardt, Eschricht, 
Kaup, Lepsius, Howes and Wilder, leave no doubt as to the fact 
that the different rudimentary structures which these anatomists 
have detected, unequivocally point to the conclusion that, the 
cetaceans and sirenians have descended from Mammalia which 
possessed more or less perfectly developed ambulatory limbs, 
which fitted them at least for an amphibious or partially terrestrial 
existence. This conclusion is, I believe, generally accepted by 
recent authorities. 
All recent writers, amongst which. may be named Flower, 
Huxley, Owen, Claus and Parker, unequivocally declare that the 
hind-limbs of the whales and sirenians have been so completely 
suppressed, that no rudiments or vestiges of any kind have re- 
mained to indicate outwardly that these creatures ever possesse 
such appendages, the evidence that they did once possess hind- 
limbs resting for them rather upon the presence ofa rudimentary 
pelvis with much reduced limb-bones in a few forms of Balznoi- 
ea and in Halitherium. eee 
From this view the-writer must dissent, having independently 
arrived at conclusions in reference to the homology of the flukes 
1 Edited by Joun A, Ryper, Smithsonian Institution, Washington, D. C. 
