26 ORGANIC EVOLUTION — PHYSICAL 



generations always the slowest, the smallest, or the least 

 milk-giving individuals respectively wherewith to con- 

 tinue the race, is there any limit to the retrogression 

 which would result in " assigned " time ? We certainly 

 should not bring about the reappearance, for instance, 

 of the remotely ancestral horse, the five-toed Eohippus, 

 for the form of the modern horse, like that of other 

 animals, has resulted not only from evolution, but also 

 from retrogression, the disappearance of the four lateral 

 toes being examples mainly of the latter, while the 

 great size of the middle toe is an example of the 

 former ; but it cannot be doubted that we should cause, 

 in a time comparatively short as compared to that 

 which elapsed during the evolution, such extreme 

 retrogression as would result in an animal quite incap- 

 able of existence. 



The improvements in our cultivated plants are the 

 result of centuries of stringent selection ; and here again 

 it cannot be doubted, that if the process by which they 

 were evolved were reversed, were we to propagate only 

 from the most inferior plants, that the rate of retro- 

 gression would be much more rapid than was the rate 

 of evolution. 



It is to be noted, however, that some cultivated 

 plants exhibit comparatively extreme evolution, and in 

 fruit or flower or other particular greatly surpass the 

 wild individuals of the species, e. g. peach, apple, pear, 

 rose, and therefore furnish apparent exceptions to the 

 law that rapid evolution is soon checked by an increas- 

 ing tendency towards retrogression. In reality they 

 afford the strongest proof of it. I think I am right in 

 saying, that in every instance such plants have been 

 propagated principally by cuttings and not by seed ; i. e. 

 they are not descendants in any true sense of their 

 immediate predecessors, but detached portions of them. 

 Their evolution appears to have been effected as 



