CHAPTER XXI 



AGE AND AREA AND THE MUTATION THEORY 

 By Hugo de Vries, F.M.R.S. 



The main principle of the mutation theory is that species and 

 varieties have originated by mutation, but are, at present, not 

 known to have originated in any other way. Originally this con- 

 ception has been derived from the hypothesis of unit-characters 

 as deduced from Darwin's Pangenesis, which led to the expecta- 

 tion of two different kinds of variability, one slow and one 

 sudden. 



Freed from the assumption of a transportation of gemmules 

 through the organism, the conception of Pangenesis is the clear 

 basis of the present manifold theories of heredity. An organic 

 being is a microcosm, says Darwin, a little universe, formed of 

 a host of self-propagating organisms, inconcei^'ably minute, and 

 numerous as the stars of heaven. In honour of Darwin, I have 

 proposed to call these minute organisms pangenes, and this name 

 has now been generally accepted under the shortened form of 

 genes. They are assumed to be the material bearers of the unit- 

 characters of species and varieties. 



This principle leads almost directly to the distinction of two 

 different kinds of variation. For the first, no material change of 

 the genes is required; they remain what they are. No two leaves 

 on a tree are exactly alike; no tAvo individuals of a species are 

 the same in exery detail. These two well-knoAvn propositions are 

 the essence of what we now call fluctuating variability. In their 

 visible features characters usually oscillate around a mean value, 

 but this does not affect their material bearers. The researches of 

 Quetelet and Galton have shown that such oscillations follow 

 the law of chance. Starting from this idea, fluctuating variability 

 of animals and plants has now become a main branch of bio- 

 logical study. 



Besides these, changes may be expected, which involve the 

 material bearers of heredity, or the genes, themselves. Some 

 may be lost, either really or apparently, and new ones may be 

 added to the stock, this latter process consisting probably in the 

 transformation of old genes into new types. In consequence of 

 such changes the external features of an organism may become 



