:S93-J 
117 [Hyatt. 
ral definition ot' tlie genus as opposed to the older and more arti- 
ficial definitions of the same taxonomic term. 
The definition of genus quoted above does not mean that one 
must necessarily follow out in time the evolution of every 
genetic cycle which can be classed as a genus. On the contrary 
it means simply that having established the occurrence of such 
cycles in any given group, and having ascertained the mor[)hic 
range of the cycle in that group, one can ajiply the knowledge 
thus gained to the estimation of the value of any series of species 
of the same group exhibiting similar rclntions, wiiether tiiese 
occur togetlier on the same level or not. 
The inference is, of course, that the forms exhibiting such a 
morphic cycle or any part of such a cycle, did come from some 
common ancestor and arise successively in time. This inference, 
however, does not exclude the holding of an opposite opinion. 
If the fact is granted that several genetic series or even one in 
a group goes through with a cycle of morphogenesis which 
accords with that of an^^ individual in the same series, there is a 
standard established and a twig of the genealogical tree can be 
clearly seen from its origin to its end formed by successive 
species. Taking this measure we can apply it to the estimation 
of the aflSnities of any similar series, whether this has l)een 
evolved slowly in time, passing through several beds or stages, or 
has arisen by the migration of species from one basin or fauna to 
another on the same geological horizon, or has been produced 
with such extreme quickness, as in the degenerative series of 
.many Ammonitinae, that all are found in the same locality and 
practically on the same horizon. 
Suppose even that it should be proven, or that some investiga- 
tors believed that such local series did really arise, as they often 
seem to do, at once from a common radical on exactly the same 
levels and simultaneously, and yet retained the aspect of a cycle, 
having some species which were phylonepionic, others phylo- 
neanic, j^hylephebic, and even phyloparagerontic. The cycle or 
part of a cycle thus formed could be united as a genus and the 
relative position of every species properly and clearly designated. 
There are some generic series of forms occurring simultane- 
ously in geologic time and in the same locality, which have hereto- 
fore been supposed to be evolved in the same way as more 
extended series, the species of which occur in succession upon 
