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. 



