cook: evolution through normal diversity 195 



of the environment, to which Darwin himself often incHned, 

 and the idea of spontaneous variation within the species put 

 forward by Meehan in opposition to Darwin. As a hfelong 

 observer, famihar with many genera and species and large 

 numbers of living plants, Meehan was competent to give testi- 

 mony and he took an important step beyond Darwin in perceiving 

 that diversity inside the species is independent of the environ- 

 ment. Nevertheless, the evolutionary bearing of diversity 

 was still obscured because Meehan shared with Darwin the 

 mistake of supposing that contrasted characters must tend to 

 disappear, through the alleged "swamping effect of intercross- 

 ing," an idea that continues to be accepted by many because 

 it appears reasonable from mathematical or statistical points 

 of view. Galton's "law of ancestral regression" has been taken 

 as a mathematical demonstration of the reality of a swamping 

 effect. 



In recent years the law of regression has been applied to al- 

 ternative inheritance as well as to quantitative or blended char- 

 acters, and no longer seems to require the assumption of an under- 

 lying tendency for the members of an interbreeding group to 

 reach a stable or uniform condition. Inheritance is seen to 

 be alternative rather than equational, as shown by intensive 

 studies of Mendelism and other methods of descent. Many 

 divergent or contrasted characters persist in hybrid populations, 

 instead of being obliterated or averaged away to uniformity. 

 Even when the hybrid offspring are closely alike in the first 

 or conjugate generation, ancestral differences may reappear 

 undiminished in the perjugate generations. 



The so-called recessive characters that can be transmitted 

 for many generations without coming into expression, as well 

 as reversions or reappearances of characters of remote ancestors, 

 afford striking evidence that transmission is distinct from ex- 

 pression, and that transmission is permanent, while expression 

 is readily changed. Phenomena of variation and diversity 

 are largely differences of expression, including accommodations, 

 or varied expressions of adaptive characters, to suit different 

 conditions of existence. 



