138 EVOLUTION 



converse. When from a single wild type 

 man succeeds in producing a multitude of 

 new varieties, we may speak of the result as 

 a progress in differentiation: but we must 

 recognize that the term is only applicable 

 loosely, and that the obvious appearance of 

 increased complexity may in reality be the 

 outcome of a process of simplification." 

 Similarly, "reversion occurs when the sum 

 total of the factors returns to that which it 

 has been in some original type." The re- 

 turn may be brought about by the omission 

 of an element or by the addition of a missing 

 element. If certain kinds of variation may 

 be called "unpacking," reversion is re- 

 packing. 



In discussing the bearing of Mendelism on 

 the theory of evolution, Bateson makes three 

 important suggestions. (1) "One has only 

 to glance over trays of birds' skins, the port- 

 folios of a herbarium, or drawers of butter- 

 flies and moths, to discover abundant ' species ' 

 which are analytical varieties of others," i.e. 

 differing in the presence or absence of defi- 

 nite factors. "The principles of heredity we 

 trace in our experimental breeding are operat- 

 ing throughout the natural world of species." 

 (2) The fact of discontinuity in variation, 

 whether it be called mutation or something 

 else, is undoubted, but hitherto there has 



