80 THE PEINCIPLES OF HEREDITY 



the cultivated type disappears absolutely from the series. It 

 does not become latent. It can never again be represented 

 in the life-history. It could only reappear as a consequence 

 of fresh progressive evolution resulting from selection similar 

 to that by which it was originally evolved. 



139. Two things, then, are evident from the foregoing. 

 First, that there is, on the average, a greater tendency towards 

 reversion than towards progression, a greater tendency to vary 

 towards the ancestry than away from it. In other words 

 there is a greater tendency to let the last steps made in the 

 life-history lapse in the development than to add other steps 

 to them. Secondly, the strength of the tendency towards 

 reversion is proportionate to the swiftness of the antecedent 

 evolution, and therefore species which have been quickly 

 evolved tend to regress gradually. For this reason it is that 

 characters long established in the species are much more 

 stable than more recent characters. In the former case, 

 reversion, to be appreciable, must be to an extremely remote 

 ancestor, whereas in the latter reversion to a much less 

 remote ancestor results in appreciable Degression. For 

 example, the cultivated pansy, which was quickly evolved 

 under Natural Selection, reverts quickly to the heartsease ; 

 but because the heartsease was very slowly evolved under 

 Natural Selection, subsequent reversion in the absence of 



France a considerable number of the best pears have been discovered in 

 woods, and this has occurred so frequently that Piteau asserts that 

 improved varieties of our cultivated fruits rarely originate with nursery- 

 men. In England, on the other hand, no instance of a good pear having 

 been found is recorded ; and Mr. Rivers informs me that he knows of 

 only one instance with apples, namely the Bess Poole, which was 

 discovered in a wood in Nottinghamshire." Piteau's statement is 

 certainly incorrect, the vast majority of improved plants having 

 originated under artificial selection. Darwin also mentions one 

 improved variety of peach as discovered in a wild place and two 

 varieties of wheat. Wheat, which is grown from seed, is of course as 

 likely to produce a good variety when it has run wild as when it is 

 cultivated. The apple and peach have been cultivated for many centuries 

 and are apt to breed true, and we do not know that the improved apple 

 and the pears found wild in woods were the offspring of reverted plants. 

 Their parents may have been, like themselves, of the cultivated type. It 

 is conceivable that the cultivated type may on rare occasions become latent 

 in reverted plants. This would mean of course that the dormant and the 

 active tendencies in the germ-plasm had changed functions, the former be- 

 coming active and the latter dormant. But, so far as the present writer 

 is aware, there is no well-authenticated instance of such an occurrence. It 

 would seem that only those characters which have been long established, 

 and are therefore strongly established, tend to become latent. Characters 

 less strongly established tend to disappear through mere regression. 



