60 ETHEL BROWNE HARVEY 
that there might be a tendency in any large group of related 
animals for the genes to segregate out according to some definite 
pattern. 
If, therefore, we make a list of all the chromosome numbers 
which have been reported for all the species of a certain class 
of Metazoa, leaving out of account results which are conflicting 
or are too old to be accurate, we find that a certain number of 
chromosomes is characteristic of that class; that is, there are con- 
siderably more species having that number of chromosomes than 
any other number. This I will call the ‘type number.’ The 
type number of a class of animals is the most frequently oceur- 
ring number and may be considered tentatively as the funda- 
mental chromosome group. One or more chromosomes of this 
group or of a group derived from it may split into two (or more) 
parts, or they may fuse, thus causing the differences in number 
which occur in related forms. Whether the double groups which 
occur in closely related forms in many plants (e. g. Oenothera 
gigas and O. lamarckiana, Drossera longifolia and D. rotundi- 
folia, Spivanthes cernua and 8. gracilis ete.) and in some animals 
(e. g. the bivalens and univalens varieties of Ascaris megaloceph- 
ala, Helix pomatia, Echinus microtuberculatus, Artemia salina; 
Cyclops viridis and C. gracilis, Anopheles sp? and Anopheles 
punctipennis ete.) are derived in all cases by a splitting of all the 
chromosomes of the.simple group, it is difficult to say. It may 
be, as suggested by Gates and supported by Strassburger that 
the double groups are derived in some cases at least, by a failure 
of cell division after the division of the chromosomes. A slight 
change in number may also be obtained by the disappearance 
of a whole chromosome, but this must be rare. All numbers 
referred to hereafter are the haploid numbers, and X, when 
present, is counted as one chromosome, even when it consists of 
several elements. 
The type number for the Coelenterates cannot be determined 
yet, as the data are too scanty and the results conflicting (e. g. 
Hydra). Possibly it is 12. For the Nemathelminthes, the type 
13 The term ‘‘class’’ is used loosely to include related families, orders or classes 
of ordinary classification. 
