26 MENDELIAN SEGREGATION 
The same manner of treatment will work for more 
than three pairs of chromosomes; the number of 
kinds of germ cells increases in geometrical ratio. 
In most animals and plants the number of chromo- 
somes is higher than in Drosophila, and the number 
of pairs of factors that may show independent assort- 
ment is, in consequence, increased. In the snail, 
Helix hortensis, the half number of the chromosomes 
is given as 22; in the potato beetle 18; in man, prob- 
ably, 24; in the mouse 20; in cotton 28; in the four- 
o’clock 16; in the garden pea 7; in corn 10; in the 
evening primrose 7; in the nightshade 36; in tobacco 
24; in the tomato 12; in wheat 8. If 20 pairs of 
chromosomes are present there will be over one 
million possible kinds of germ cells in the F, hybrid. 
The number of combinations that two such sets of 
germ cells may produce through fertilization is 
enormously greater. From this point of view we 
can understand the absence of identical individuals 
in such mixed types as the human race. The chance 
of identity is still further decreased since in addition 
there may be very large numbers of factors within 
each chromosome. 
