222 Venous System of Didelphys Marsupials (LL) 
bryonic and adult condition of the postcava in Echidna (see Text Figs. 
x and 2 Part: L). 
Furthermore, two cases were met with among the 101 adults exam- 
ined in which the postcava occupied the same relative position with re- 
spect to the aorta and was formed through a union of its iliac tributaries 
in exactly the same manner as in placental mammals when the left in- 
stead of the right postcardinal vein forms the caudal end of the stem of 
the postcava. These two cases are regarded by the writer as the only 
actual cases of abnormalities met with among the 101 adults examined. 
They are figured and described in Part I (page 395) and further con- 
sidered from the standpoint of their development in Part II (p. 211). 
It is impossible to state what the causes may be which are responsible 
for such a series of normally occurring variations as those which charac- 
terize the postcava of the adult Didelphys. Whatever they may be, they 
are apparently inherent in the individual itself since there is as much 
variation among the individuals of the same litter as among the in- 
dividual members which constitute the species in general. 
‘ It appears to the writer that the causes which account for this constant 
variation in Didelphys may be analogous to those which only sporadically 
act in other vertebrates, but which are responsible for the well-known 
series of venous abnormalities which one occasionally meets with in the 
adult. Why they should constantly act in Didelphys and only ocecasion- 
ally act in other vertebrates, however, is difficult of explanation unless it 
is indicative of an extreme plasticity which may be regarded as charac- 
teristic of the species in general. This view certainly coincides with the 
investigations of Allen, or, upon the variable character of the skeleton 
of Didelphys, as well as with those of Oldfield Thomas, 88, who in re- 
ferring to Didelphys marsupialis, var. typica, savs (p. 327) : “ This wide- 
spread species, owing to its remarkable variability in color, has been made 
the basis of a very considerable number of nominal species, of which the 
most commonly recognized are the North American D. virginiana, the 
Brazilian D. cancrivora, and the striped-faced D. azare. I find, how- 
ever, such a considerable amount of variability in the specimens from 
every locality and such an entire absence of constancy in any character 
or set of characters, that I am constrained to unite the whole of this group 
of opossums into a single species, to which the Linnean name D. marsu- 
pialis is of course applicable.” 
It is generally conceded that variations of the venous system oceur 
with greater frequency among domesticated animals than among those 
living in the wild state: an idea, however, which is most certainly erro- 
neous, as proved by the conditions met with in Didelphys. 
x 
