INTRODUCTORY. 



5. Species-formation \K chancewise in direct .ion, QI ilting from a trnoVm-y 



to vary in any one determinate direction. 



6. Natural selection can not give origin to new species; it ran only \u-e.l out from 



those already in existence such as are incapable of sustaining t hem-el ves. 



The central conception of this whole theory of mutation is that of fixed unit- 

 characters. Specific characters are called units, because they are supposed to be 

 individualities and to remain such in all stages of development and evolution. If 

 such a character undergoes change, it ceases to be the same character and by 

 sudden transformation becomes an entirely new unit, or unit-character. Trans- 

 form one of these units, add one, or remove one, in each case the old species beco 

 a new species per saltum. These units are imagined to have the clean-cut individ- 

 uality of a chemical molecule, and any real change is likened to a chemical 

 substitution. 



Such views are captivating, for they offer definiteness in place of the vague 

 and mysterious. The problem of the origin of species hangs on the nature and 

 meaning of specific characters. Reveal the secrets in the development and evolu- 

 tion of a single specific character, and you will furnish the key for understanding 

 all characters and all species. Right or wrong, deVries has done well to direct 

 attention to the all-important point, the specific character. Here. 1 believe, lie- 

 the whole problem of evolution, and de Vries is right in trying to draw a line 

 between specific and non-specific variations. 



It should not be overlooked, however, that Weismann has already set us far 

 along in this task; for not long ago he convinced most of us that only germinal 

 variations are hereditary and that all so-called "acquired characters" of somatic 

 origin are non-transmissible and therefore without specific .significance. 



Do we not have, then, in germinal variation, a better criterion of what is specific 

 than we get in sudden appearance? Indeed, is it not here that the seeming sudden- 

 ness of first appearance finds its explanation, and likewise the fact that so-called 

 mutations involve the whole organism? If we are to accept the physiological con- 

 ception of development, as is inevitable in my opinion, it is easy to see that a 

 change, however slight, in the primordial constitution of the germ would tend to 

 correlate itself with every part of the whole germ-system, so that the end-stage of 

 development would present a new facies and appear as a total modification, answer- 

 ing to what de Vries would call a mutation. That something of this order does 

 sometimes occur I have indubitable evidence, and in such form as to dispel the idea 

 of discontinuity and sudden gaps in transformation. 



The idea of unit-characters, however, as distinct elements that can be removed 

 or introduced bodily into the germ does not appeal to me as removing difficulties. 

 but rather as hiding them; in short, as a return to the old pangenesis view of 

 preformed characters. In this theory, as is well known, we had two miracles in- 

 volved. The first consisted in a centripetal migration of preformed gemmules and 

 the second in the centrifugal distribution of the same elements. De Vries dismi 

 the first of these, but accepts the second, and on it rears the superstructure of his 

 theory of mutable-immutable unit-characters. With all due respect to the dis- 

 tinguished author of this theory, and with abounding admiration for his great work 

 and model methods, which have aroused universal interest and stimulated enor- 

 mously experimental bionomics, I am strongly persuaded that his hypothesis of 

 unit-characters fails as a guide to the interpretation of the species and its characters. 



