HEREDITY IN PURE LINES 377 
Johanssen pointed out that each pure line had a different mean 
size of bean and a different distribution about the mean. This was a 
real hereditary difference due to differences in the germinal content 
of the original parent-beans. Two beans of exactly the same size, one 
an average individual of a larger stock and one a large individual of a 
smaller stock, were planted and their offspring varied about two dis- 
tinct means. This leads to the idea that an individual produces off- 
spring not in accord with its somatic appearance, but according to its 
germinal content. So this idea of the difference between what an 
individual is somatically, and what it is germinally led Johanssen to 
introduce the terms “phenotypic” and ‘genotypic’ —‘‘ phenotype” 
and “genotype.” 
Thus, if one selected all the beans of a given size he would have a 
group of phenotypes that would be identical phenotypically, but would 
be very different genotypically; for each might be germinally different 
and would therefore have different groups of offspring. All of the 
individuals in one pure line, however, whether they differ somatically 
(phenotypically) or not, would belong to the same genotype and would 
be genotypically equivalent. 
The appreciation of this distinction at this place, before the treat- 
ment of Mendelian heredity, will be of great advantage. Few more 
useful terms have been devised in connection with genetics than geno- 
type and phenotype. 
W. L. Tower in a long series of experiments on the potato beetle 
(Leptinotarsa decemlineata) came to similar conclusions as the result 
of his attempts to modify a character by selection. Instead of using 
a self-fertilizing type, he chose a long inbred stock that was probably 
all identical germinally, but varied considerably in shade of color, etc. 
He selected for twelve generations the darkest specimens and, instead 
of getting all dark offspring, he got an array of all shades fluctuating 
evenly about the average. At the end of twelve generations of 
selection there was no change in the proportion of light and dark 
individuals. 
Jennings tried another type of pure-line work, using one-celled 
organisms which reproduce by binary fission, i.e., by the division of the 
parent into two nearly equal halves, thus forming two offspring. He 
chose a considerable number of Paramecia and isolated each in a 
separate small aquarium where it was allowed to breed for some gen- 
erations. The original individuals differed quite markedly in size and 
in other structural characters. The various sets of progeny were 
