STUDIES ON CHROMOSOMES 105 
a fact (Lutz, ’07; Gates, ’08), though it may be due in this instance 
to a simple doubling of the whole group. Such cases led me sev- 
eral years ago to the conclusion ‘“‘that the nucleus consists of 
many different materials that segregate in a particular pattern 
and that the particular form of segregation may readily 
change from species to species”? (Wilson, ’09d, p. 2). 
Such changes must involve corresponding ones in the morpho- 
logical and physiological value of the individual chromosomes; 
and we must accordingly recognize the probability that these 
individual values, though constant for the species, may change 
from species to species as readily as does the number. Despite 
the conformity to a-general type often exhibited by particular 
genera or even by higher groups, the individual chromosomes are 
therefore per se of subordinate significance; and it may often be 
practically impossible to establish exact homologies between those 
of different species. How closely this bears on the origin of the 
diverse conditions seen in the composition of the XY-pair is 
obvious. 
Lastly, it is almost certain that changes of number may some- 
times arise as a result of abnormalities in the process of karyoki- 
nesis, such as the passage of both daughter-chromosomes, or of 
both members of a bivalent, to one pole. In Metapodius I found 
(096) direct evidence of this in case of the XY-pair itself, and 
endeavored to trace to this initial cause the remarkakle variations 
of number that occur in this genus. Many other observers have 
recorded anomalies of this kind, in both plants and animals. It 
seems entirely possible, as has been suggested by McClung (’05) 
and by Gates(’08)that definite mutations may be traceable to this 
cause; though probably such abnormalities may in general be 
expected to lead to pathological conditions. 
CONCLUSION 
Some of the suggestions offered in the foregoing discussion are 
admittedly of a somewhat speculative character; but they are not, 
as I venture to think, mere a priori constructions, but are forced 
upon our attention by the observed facts. The time has come 
JOURNAL OF MORPHOLOGY, VOL, 22, No. 1 
