58 ETHEL BROWNE HARVEY 
closely related species may have widely different numbers, and that 
the numbers in related species are not usually in arithmetical ratio 
although occasionally they are, especially in plants. It is also 
apparent that numbers which are resolvable into factors of 2 and 
3 are of frequent occurrence, as one would expect since nearly half 
of the numbers between 2 and 20 (the most frequently occurring 
numbers) are resolvable into these factors. However, it is equally 
apparent that other numbers not resolvable into these factors are 
also of frequent occurrence. 
In using the present tabulation for any generalizations or con- 
clusions, several facts must be taken into consideration. Many 
of the observations recorded are of too early a date to be of much 
value. Other observations are contradictory and in many cases 
it is impossible to judge which is correct; this is largely due to 
difficult material and is especially true for the mammals, where 
for man the number of chromosomes varies between 8 and 48 
(diploid) according to different authorities. 
IV. CHROMOSOME NUMBERS 
In looking over the fore-going list, there can be no doubt to 
an unprejudiced mind that the constancy of chromosome 
numbers for a species is a fact, and that any variation in number 
for a definite species is an exception to the general rule. Such 
variations occur regularly in Notonecta insulata, Jamaicana uni- 
color and J. subguttata, and Hesperotettix viridis where two or 
more chromosomes may be united or separate; in species with 
supernumeraries (see p. 66); in cases where multiple groups occur 
(e. g. Culex pipiens, Notonecta, Anasa), and where fragmenta- 
tion has taken place (e. g. Ascaris, pig). A few sporadic varia- 
tions occur in certain species owing to the lack of conjugation of 
two univalents (e. g. Lygaeus turcicus, Coenus delius, Euschistus) 
and a few which have not been explained (e. g. Trichopepla, 
Lygaeus reclivatus which is now under investigation). When a 
range of numbers is given instead of one definite number, it is 
usually due either to the early date at which the observation was 
made or to difficult material rendering accurate counting 
impossible. 
