ORGANIC EVOLUTION 283 



ment, for we must always remember that there can be no 

 selection of single characters; it is individuals that are 

 selected, with whatever combination of characters they may 

 happen to be endowed. If the fittest Irish elk had ever 

 antlers of increasing size, the only possible curb to antler 

 development would lie in the extermination of the line. 

 Natural selection can affect an organ only when that organ 

 causes such manifest unfitness in the organism as is incom- 

 patible with the conditions of racial existence. 



The phenomena of orthogenesis indicate that the springs 

 of genetic progress lie very deep and that we must look for 

 the origin of species in the origin of variations and of develop- 

 mental tendencies. This matter will be considered a little 

 further in the next chapter. 



Segregation. The breeder of plants or of animals isolates 

 his choice varieties (except when propagated asexually) in 

 order to obviate the retrogression that would inevitably 

 result from intercrossing with inferior varieties. Biparental 

 reproduction necessitates this. Nature also segregates her 

 new forms more or less rigidly, and by a great variety of 

 means, among which may be mentioned both external and 

 internal agencies. 



i) Geographic barriers. Two closely allied species, 

 whose differentiation from one another may have been 

 comparatively recent, are often found on opposite sides oi a 

 mountain chain or desert, or other impassible barrier. 

 Thus most of the fishes found on the two sides of the Isthmus 

 of Panama are represented by two closely allied species, one 

 on one side and the other on the other side. This is held to 

 confirm the opinion of geologists, that the two oceans were 

 once connected across the isthmus by open sea, the assump- 

 tion being that time enough has elapsed since the emer- 

 gence of the Isthmus, closing the passage, for the differentia- 



