Hyatt.] 118 [April 5, 
dit't'rirnt levels; tluTO are also series of laroor liroiips arising 
simultaneously, spreading out like the spokes of a fan from a 
eommon form. Tliat cycles may have been formed in this way 
through the action of the surroundings upon a common organism, 
either by simultaneous genesis of forms or so closely simultaneous 
that the element of succession in time was oT)literate(T, is 
a statement that can he sup|)orted by a certain series of facts of 
observation. Although one should be wary of admitting such a 
view, it is a thesis which can be well sup])orted, and in fact has 
been directly maintained by Neumayr and othei-s in the theory 
of polygenesis.^ 
If it were to be established that a cycle could be thus generated, 
it would not invalidate the definition here advocated. The only 
fact that would make it useless, so far as the genus is concerned, 
would be the discovery of |)hylocycles in the species, because in 
this definition it is said to he "the smallest natui-al group which 
is genetically connected, and in which a more or less complete 
cycle of forms or s[)ecies may be traced." 
Professor Blake's nomenclature, however, so far as he has 
sketched it, certainly does point to a possible imi)rovement in the 
explanatory nomenclature of groups which would l)e of great 
assistance, although not, as suggested by him, of such ]irimary 
importance as to form a good substitute for the estal)lished cate- 
gories, genus, species, etc. 
The application of definite popular terms to certain subdivisions 
which would at once explain their significance in the evolution of 
the whole group to which they miglit belong, would be extremely , 
convenient. Thus, among the Arietidae I found it useful to call 
the common point of origin of the family, the genus Psiloceras, 
"the radical stock," and the different genera that sprang from 
this in radiating lines according to their general genetic relations 
by other names ; the plicatus stock included the genera 
Waehneroceras, Schlotheimia, Caloceras, and Vermiceras, and the 
levis stock included the genei'a Arnioceras, Coroniceras, Agassi- 
ceras, Asteroceras, and Oxynoticeras. 
In order to make my meaning with regard to these groups 
still clearer, I also defined each genus by the word series. Thus, 
under each stock in the general descriptions of the genetic rela- 
1 See Genesis of the Arietidae, p. 118. 
