24 THE POPULAR SCIENCE MONTHLY 
of this possibility, we are not as yet entitled to explain all blending away 
as illusory; but we may bear in mind that this may be the case. It can 
at least be said, that scarcely a month passes without some case of 
inheritance, formerly seeming inscrutable, being brought into the field 
of well-ascertained law. 
With the incoming of the idea of unit characters, passes our former 
conception of continuous variability. Supposing every character to be 
at all times variable—that is in motion, as it were, away from its 
present center of stability—there is no doubt that continuous selection 
would be required to keep characters up to any particular standard. 
The extraordinary permanency of some organic characters should suffice 
to make us doubt this necessity. For millions of years, certain features 
in the lower animals have been handed down generation after genera- 
tion, practically without change. When we remember the tremendous 
complexity of the protoplasm molecule and the much greater complexity 
of the least imaginable bearer of heredity, and the fact that it has not 
been possible to break up and then reform the combination, as in 
inorganic chemistry, the permanency of these units in time is simply 
amazing. Least particles of protoplasmic jelly, they have stood while 
the rocks have been ground to dust, and made over many times. They 
are entitled to be ranked among the most permanent things in nature. 
What then of the facts of variability, as they appear to us? What 
is the use of denying continuous variability, in the face of the fact that 
no two human beings are alike? The paradox may be resolved, when 
we remember the extraordinary number of words in the English lan- 
guage, no two the same—yet made up of the undeniably unchanging 
letters of the alphabet. When we recall that, on the unit character 
theory, the units in man must be exceedingly numerous, and must be 
recombined in almost every conceivable way in bisexual inheritance, it 
is easy to see that the chances against any two individuals coming out 
exactly the same are so great that such a result is practically impossible. 
The only case which can come under this head are those of identical 
twins, where the resemblance is indeed amazing, throwing light on the 
extraordinary potency of inheritance. Such twins are believed to result 
from the division of a single fertilized ovum, and hence to be, in a 
biological sense, two halves of a single individual. 
Much light has been thrown on the permanence of unit-characters 
by studies among plants and protozoan animals of what are called pure 
lines. A pure line is one in which all the individuals have the same 
ancestry, uncontaminated by crossing. The most remarkable results 
have been obtained by Professor Jennings in his studies of Paramecium. 
He says: 
In a given “pure line” (progeny of a single individual) all detectible 
variations are due to growth and environmental action, and are not inherited. 
