334 THE AMERICAN. NATURALIST. [Vor. XXXVI. 
species belonging to old and widespread genera, pointing to a recent 
period of plasticity of types, not going so far, however, as to affect 
the genera. For the development of new genera, environmental con- 
ditions of great stability and complexity, such as exist to-day in the 
tropics, seem to be most favorable; and it may very well be that 
most of the distinctive North American genera had their origin at a 
time long previous to the glacial period, wlien such conditions more 
nearly prevailed on this continent. The changeable conditions of 
recent periods, while extremely favorable to the production of new 
subspecies or even species, might from their very instability be inimical 
to the development of new genera. 
It is interesting to observe how much more plastic some species 
are than others. Thus, in the genus Melospiza, M. Zimcoinii and 
M. georgiana, though: very widespread, remain monotypic, while 
M. cinerea has split up into no less than twenty-one subspecific 
forms, many of very restricted distribution. 
A very interesting discussion is given of the Galapagos genus 
Geospiza. This includes twenty-seven forms, the extremes of which 
are so distinct that they used to be assigned to different genera; yet 
the gradation from one to the other is almost complete. Messrs. 
Rothschild and Hartert proposed to regard a number of these forms 
as subspecies of the others; but Mr. Ridgway urges that there is 
really no alternative between regarding them all as forms of one 
species and treating them all as specific units. Since the former 
course is one that would commend itself to no one, the latter is 
adopted, with the result that we have a series of “ species " far more 
intimately related than the forms usually classed in this category. 
We feel much as we might feel if suddenly confronted with the 
whole series of extinct types connecting our very *good" genera 
and species of the present day. Were the Galapagos less isolated, 
and the competition with other groups of birds more severe, no 
doubt several of the links would be conveniently missing by this 
time, and we should not have occasion to dispute over the status 
of the remainder. - CR ue miM i : 
Many endemic forms are recorded from the islands of the Cali- 
fornia coast and the West. Indies. These are in most cases regarded 
as subspecies only ; though, in the inevitable absence of intermediate 
series occupying the intervening territory (this being water), it would 
seem more logical to treat them as distinct species. We seem to need 
rm to express these allied insular forms ; but since their conti- 
_nuity with the parent species is assuredly broken, I would rather write 
ane ‘ae 

VELIM LU 
i3 

