302 MONTGOMERY. [VoL. XIT. 
fication, and regressive development to a structural simplification 
of the organism. Now, as is generally conceded, species are 
maintained in the struggle for existence by the preservation of 
favorable individual variations, z.e. (to my mind), variations 
which are favorable, in as much as they tend to produce a 
complete correlation of the organs; and all such structural 
variations are necessarily either more complex or more simple 
than the normal. The preservation of favorable individual 
variations which are more complex would result in the produc- 
tion of a more highly differentiated species ; and, on the other 
hand, the preservation of those which are less complex would 
result in the formation of a morphologically less differentiated 
species. Accordingly, we must first determine whether the 
variations are above or below the normal, — more complex or 
more simple. For if the variations are more complex, then 
if they should be preserved a more highly organized species 
would be evolved ; and if they are structurally simpler, and 
should be preserved, a less highly organized. Similarly, judging 
from the paleontological remains of a series of individuals of 
a now extinct species, it might be possible, after a careful investi- 
gation into the nature and amount of individual variations 
exhibited by them, to conclude whether a more highly or a less 
highly organized form, if any, had been produced. Thus we 
should expect, that a given species A occurring in the Liassic 
beds, presenting individual variations (osteological, e.g.) more 
complex than the normal, would be represented in the Triassic 
by a more highly differentiated species, if by any. 
But the study of variation, thus far considered, gives us 
criteria for only the future development of the species, so that 
it remains necessary to seek criteria from the phenomena of 
variation for the past phylogenetic stages. 
And, firstly, it is desirable to determine as far as possible 
what limits there are to individual variation. It seems to be 
well ascertained that there is a limit to such variation, but 
where that limit may be placed for a given organism, or what 
organic law fixes it, is very difficult of experimental proof. 
Since we find the amount of variation to be in direct proportion 
to the amount of change in the environment, the question is, 
