EVOLUTION, CYTOLOGY AND MENDEL'S LA^VS. 227 



hand, characters acquired through inbreeding or other debilitating 

 causes may disappear or become recessive as soon as crossing permits 

 a return to a more normal and vigorous ancestral type of organization, 

 as in the historic pigeon experiments of Darwin. The popularization 

 of Mendel's laws should make it more easy to perceive that the normal 

 effect of cross-breeding is a progressive synthetic evolution and not 

 a stationary average, though we are having some fine examples of the 

 lengths to which the specialist will sometimes go to escape facts too 

 simple and obvious for his appreciation. 



Individual 'Hybrids.' — Perhaps the loosest use of the word hybrid 

 is for the offspring of crosses between so-called 'horticultural varieties' 

 of domesticated plants propagated by cuttings or grafts. Everybody 

 knows, though some forget, that the Baldwin apple, the Bartlett pear, 

 the Niagara grape, and a great multitude of analogous sorts, are de- 

 scending from single seedling trees or vines, and are thus for evolu- 

 tionary purposes single individuals. The distinction between such 

 individuals and those of wild species in nature is largely psychological ; 

 we have learned to regard differences between individual apple trees, 

 but have not attained such close acquaintance with oaks and elms. 

 If crosses between the normally diverse individuals of a species are 

 to be termed hybrids then the word covers all sexually differentiated 

 organisms and is utterly useless as a means of drawing biological dis- 

 tinctions. Mendel deliberately disregarded the question as to which 

 of his pea hybrids were between different species, and which between 

 varieties merely, and for the purposes of his inquiry this was a matter 

 of little importance. But for his followers to draw general conclu- 

 sions, while ignoring all distinction between the evolutionary condi- 

 tions of the organisms which they study, is a reversion to the same 

 general woolliness of evolutionary thinking to which Mendel consti- 

 tuted so brilliant an exception. 



The millions of species with which nature has been experimenting 

 for millions of years seem to make it very plain that individual diver- 

 sity with free interbreeding is the optimum condition for evolutionary 

 progress, since this is what we find everywhere among natural species. 

 It is true that the diversity masks the slow and gradual motion of the 

 species from perception by our momentary observations, and also that 

 the interbreeding hinders the segregation of species; but we may take 

 the results as evidence that evolutionary progress is not impeded by 

 wide individual variation, nor by opportunities for the progressive 

 accumulation of new characters. Nor need we turn our backs on this 

 interpretation of the history of organic nature because Mendel and 

 others have given new demonstrations of the old fact that there are 

 degrees of evolutionary divergence in which the combination of parental 

 characters is no longer possible. 



