THE VITAL FABRIC OF DESCENT 315 



members of the same species. Mutation is a reaction from the 

 abnormal uniformity which is the first effect of selective inbreed- 

 ing. Not only do the same or closely similar mutations occur 

 repeatedly in the same species, but different species and genera 

 may mutate in the same way, just as the same disease may call 

 forth similar symptoms in different plants or animals. But 

 even in this respect mutations may be looked upon as furnishing 

 indications of the behavior of normal variations. Species, like 

 other bodies, can move only from where they are; each " new 

 character" is, after all, only a modification of parts already 

 existing. The novelty is very largely that of the language in 

 which it is described. Genetic variation is not completely inde- 

 terminate, fortuitous or in all directions at random ; nor is it 

 narrowly determinate or limited to one character, or two char- 

 acters, or to any small number of characters, as we well know 

 from the excellent example of individual diversity afforded by 

 the members of our own species. Variation does of necessity 

 have reference to characters already existing, and must be con- 

 sistent with these if the change is to be advantageous. Some 

 varietal or racial characters are also prepotent over others, and 

 with sufficient opportunity of interbreeding will continue to 

 spread, and to become more and more accentuated. 



It is therefore in accordance with the most obvious probabili- 

 ties of kinetic evolution that nature should abound in instances 

 of parallel development.^ The same or similar variations are 

 likely to arise more than once and to have a similar welcome or 

 rejection by characters already existing. Tendencies of varia- 

 tion once begun in a species are continued, even after the spe- 

 cies subdivides. Each natural group, of whatever rank, was 

 once a single interbreeding species, and every such group rep- 

 resents, in evolutionary history, the subdivision of an original 

 species. Each character or tendency can continue its develop- 

 ment, though in the company of different later variations in 

 each of the groups, as they have successively segregated. The 

 static theories, which ascribed evolution to environment, might 



1 Instances of parallel development have been reviewed recently by Professor 

 Osborn as affording "evidence of a predisposition to similar evolution." (Sci- 

 ence, N. S., 21 : 28, January 6, 1905.) 



Proc. Wash. Acad. Sci., March, 1906. 



