B. G, Wilder on a fetal Manatee and Cetacean. 111 
or groups, primarily involves an increase in complexity and an 
advance in rank; but it is perfectly compatible with a sub- 
sequent retrograde metamorphosis, more or less extensive ac- 
cording to the rank already attained. Using the terms in their 
literal rather than their usually accepted sense, evolution pri- 
marily involves ascent, but it is perfectly compatible with de- 
scent, 
Upon the derivative hypothesis all evidences of a parallelism 
or concomitancy between individual metamorphosis and the 
evolution of types toward a more perfect condition are equally 
cogent in favor of the conclusion that the retrograde metamor- 
phosis of an individual indicates that the group to which it be- 
longs is upon the downward rather than the upward path. 
Upon the hypothesis of evolution we may regard exceptions 
to the rule quoted from Dana as to the resemblance between 
the earlier stages of higher forms and the permanent condition 
ower, as strictly in accordance with the more universal 
law that the earlier stages of animals resemble their more or less 
remote ancestors ; if these are lower in rank, as is usually the 
case, then the commonly accepted rule will hold good; but 
if higher, as is here suggested of the Sireni:, then that rule would 
. . 
fail, while the more universal one would still be kept. 
urges against the ordinary view of progressive development. 
He says: “To the supposition that Halitherium has given rise 
to the other and later genera are 
Oppo . , 
For the Halitheria, in addition to the well-developed pelvis 
f Deinotherium* be regarded as a Sirenian with limbs yet 
more developed then in Halitherium, then the series is at least 
provisionally intelligible. 
Of course the above considerations in no way account for 
the prior existence of the hypothetical stem-form of quadrupeds 
.* By Owen (12, ii, 282); Huxley (13, 431); Haeckel (14, 560); Brandt (2, 190); 
Gn (35, 9h De Dainucdae io oat aes iawig tien but it is in- 
35 among the Sirenia by De Blainville and St. Hilaire (Murray, 3, 198), Dana, 
Hu Murray, 3, 190, and L. Agassiz. According to the placental classification of 
thowey and Haeckel, idia are in one great group Deciduata, w 
Sirenia are in the other, but provisionally, since their placentation is not 
Sg2,, Tt would seem that osteological comparisons are not as yet conclusive in 
