April 12, 1907] 



SCIENCE. 



591 



maintain themselves absolutely distinct. This 

 should convince any one who may still enter- 

 tain any doubt regarding it that in the pres- 

 ence of Mendelian hybridization, no form of 

 physical isolation is necessary for the main- 

 tenance of closely related forms. Moreover, 

 these observations on Bursa show that Mende- 

 lian behavior is a strictly normal natural 

 process and in no manner dependent upon 

 the artificial conditions supplied by garden 

 practise. 



When a new form arises, differing from the 

 parent in one or several unit characters, these 

 new characters may be either dominant or 

 recessive to the corresponding character of the 

 parent. Less rarely they are neither domi- 

 nant nor recessive. The chances of survival 

 under these several possible conditions seem 

 to need discussion, since, in several recent 

 conversations, I have found the notion to pre- 

 vail that recessiveness is a handicap, and allu- 

 sions based upon the same idea have found 

 their way into print. This view is quite 

 erroneous; not only has the dominant form 

 no advantage in the competition which the 

 newly arisen elementary species must en- 

 counter, but it can be shown that under cer- 

 tain conditions the reverse is true. 



If the dominant and recessive forms are 

 equally adapted to the particular environment 

 in which they live, there is absolutely no ad- 

 vantage in favor of either. The second gen- 

 eration of a Mendelian monohybrid contains 

 the same number of pure reeessives as of pure 

 dominants, and the heterozygotes continue to 

 produce in each succeeding generation Just as 

 many reeessives as extracted dominants. The 

 chances that extracted dominants will self- 

 fertilize or that they will cross with other 

 extracted dominants are exactly the same as 

 the chances that reeessives will self-fertilize 

 or cross with other reeessives. In like manner 

 extracted dominants and reeessives will cross 

 with heterozygotes with equal frequency, and 

 the quantitative results in these two cases will 

 be exactly parallel, in one instance giving 

 fifty per cent, of pure dominants, in the other 

 case fifty per cent, of pure reeessives. In this 

 equal fashion the struggle will continue in- 

 definitely so long as the premise holds, that 



the two forms are equally well suited to the 

 conditions under which they must grow. 



The situation is different when natural 

 selection favors one or the other of the com- 

 peting forms. A single extreme case will 

 suffice to demonstrate : Let us suppose that the 

 new form is dominant over its parent, but 

 so poorly adapted to the particular habitat in 

 which it originated that it can not successfully 

 compete with the parent form. All the 

 hybrid offspring resulting from crosses be- 

 tween the mutant and its parent will have the 

 unadapted new form, and when the selection 

 becomes extreme, not only will all the pure- 

 bred specimens of the new form be destroyed, 

 but all the hybrids as well, and in this way 

 every vestige of the new form will be entirely 

 lost. Assuming, on the other hand, that the 

 mutant is recessive to its parent but that in 

 other respects the conditions are the same as 

 before, the extreme selection that is assumed 

 to destroy all the recessive individuals, leaves 

 the heterozygotes living because they have the 

 successful form possessed by the parent 

 species. These successful heterozygotes give 

 rise to a progeny in the next generation in- 

 cluding the recessive form, and also a con- 

 siderable percentage of heterozygotes that may 

 carry the form on to still another generation, 

 and in this way the recessive mutant may be 

 preserved indefinitely under the protection of 

 the dominant characteristics of its more suc- 

 cessful parent. Such prolongation of the life 

 of a recessive may serve to tide it over times 

 of special stress, or may continue its exist- 

 ence until the various distributing agents 

 have carried it beyond the limits of the habitat 

 in which it is a, failure into others in which 

 it may become a success. 



George H. Shull 



Station fob Experimental Evolution, 

 Cold Spring Hakbor, L. I. 



NEW PROCESSES OF TAKING IMPRESSIONS OF 

 NATURAL MOLDS OF FOSSILS 



One of the perplexing problems which con- 

 front the invertebrate paleontologist is that 

 of finding some substance with which an ac- 

 curate and permanent cast or impression of 

 natural molds of fossils can be made. In 



