32 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Basis of genus: many external characters, main divisions of j^iws 
easily homologized, fourth ventral plates related, forceps related. 
Specific characters: feio external characters, clasjiers, differences in 
parts of fourth ventral plates, forceps in part. 
For the purposes of discussion it is assumed that both Ra^ inia and 
Boettcheria are genera derived from Sarcophaga. Whether this be so 
or whether all three genera are descended from a more primitive form 
does not matter, the ideas set forth are equally applicable either way. 
In connection with their relationship it might be said, however, that 
in certain characters several species of Sarcophaga resemble Ravinia, 
notably members of the assidua group. Sarcophaga scoparia Pandelle 
bears a general resemblance to the species of Boettcheria. But pro- 
viding these two genera actually did branch off from Sarcophaga, these 
suggestions are not made as even possibly indicative of species carry- 
ing forward characters represented in the primitive line of descent, 
but merely to show that a certain relation exists from a genetic stand- 
point. Indeed, it might be mentioned that two of our species of 
Wohlfartia have penes which more nearly resemble those of Ravinia 
than do any found among the species of Sarcophaga. Also, the writer 
has among his material a species, which in many ways 'resembles 
Ravinia and Wohlfartia, seeming to form a connecting link. 
The conclusion to be drawn from the evidence which has been pre- 
sented is that the genera Boettcheria and Ravinia must be defined as 
such in terms of characters which were specific in the parent genus 
Sarcophaga, of which a part, not all, of the generic characters have 
been retained. Or to state it differently, among these closely related 
genera no one set of characters can be generic and another entirely dif- 
ferent set specific ; they are sometimes one, sometimes the other. To 
bring out this point it was shown that within Sarcophaga, as it now 
exists, there are species groups, each defined by definite characters, of 
which a part may appear in one or more of the other groups both as 
group and as specific characters. Species groups, if natural, cannot be 
otherwise considered than as latent genera, which in due course of time 
will sufficiently differentiate themselves from other groups of the par- 
ent genus to become distinct, after all connecting gradations have 
disappeared. Certain it is, then, that since these groups are defined 
in terms of a part of the specific characters of the genus Sarcophaga, 
they will, as genera, be defined by a part of the same characters, some 
perhaps having been lost, others added. But these characters will 
