Isolation. 37 



the well-known fact that all structures vary more 

 or less round a specific mean as proof that natural 

 selection need not be incommoded by free inter- 

 crossing, but can of itself produce all the known 

 phenomena of specific evolution, he fails to perceive 

 that his argument refers only to one aspect of such 

 evolution (viz. the transformation of species in time), 

 and does not apply to the aspect with which alone 

 my paper on Physiological Selection was concerned 

 (viz. the multiplication of species in space). 



The same thing may be shown in this way. It is 

 perfectly obvious that where the improvement of type 

 in a linear series is concerned (monotypic evolution), 

 free intercrossing, far from being a hindrance to the 

 process, is the very means by which the process is 

 accomplished. Improvement here ascends by suc- 

 cessive steps, in successive generations, simply because 

 of the general intercrossing of the generally most fit 

 with the result that the species, as a whole, gradually 

 becomes transformed into another species, as a whole. 

 Therefore, It would be mere fatuity in any one to 

 adduce free intercrossing as a "difficulty" against 

 natural selection alone being competent to produce 

 evolution of this kind. But where the kind of 

 evolution is that whereby the species is differentiated 

 — where it is required, for instance, to produce different 

 structures in different portions of the species, such as 

 the commencement of a fighting spur on the wing of 

 a duck, or novel characters of any sort in different 

 groups of the species — free intercrossing is no longer 

 a condition to, but an absolute preventive of, the 

 process ; and, therefore, unless checked as between 

 each portion of the species by some form of homo- 



