148 SUMMARY AND CONCLUSION. 



tion of some existing species or the setting apart of new groups that 

 grow into new species unless reabsorbed by crossing or exterminated 

 by competition. 



The whole process of bionomic evolution, whether progressive or 

 retrogressive, whether increasingly ramified and divergent, or increas- 

 ingly convergent through amalgamation, is a process by which the 

 limitations of segregate breeding are either set up and established or 

 cast down and obliterated. But, as we have already seen, on the side 

 of amalgamation an impassable barrier is in time reached in the 

 physiological and psychological incompatibilities of long-established 

 types, while on the side of advancing segregation the possibilities are 

 constantly increasing. The general result is that new isolations 

 and incompatibilities are constantly arising, forming new races and 

 species, which in time become so divergent that it is impossible for 

 them to coalesce under any conditions. 



2. Unbalanced Propagation. 



If we wish to find a principle which, if continued from generation to 

 generation, will steadily tend towards the transformation of type, it 

 is unbalanced propagation continuously of the same sign. That is, 

 if the result desired is increase of the character under consideration, 

 the selection in successive generations must be of those individuals 

 that possess the character in more than the average degree ; and such 

 selection may be said to be continuously of the plus form. If the in- 

 dividuals selected in each generation depart from the type, but are so 

 selected that those above the average are exactly sufficient to balance 

 those from below the average, the average character of the mixing 

 mass will be the same as the average of the original stock ; and again, 

 if the selection is plus in one generation and equally minus in the next 

 generation, the result will be uncertain, even though long continued, 

 for the effects of selection in one generation will be balanced by the 

 effects of selection in the next generation, and we shall have one form 

 of balanced selection. With a definition of balanced propagation that 

 includes balancing of both the kinds just mentioned, we may say with 

 confidence that unbalanced propagation, if continuous, will produce 

 transformation, and that balanced propagation of the type, if con- 

 tinuous, will produce stability of type, and that balanced propagation 

 of forms, some of which are considerably above the type and others 

 of which are considerably below the type, will produce fluctuating 

 variation. 



We may next ask, how is unbalanced propagation brought about ? 

 The answer is that, in both natural and artificial breeding, it may be 



