HEREDITY, AND THE ORIGIN OF SPECIES— MACDOUGAL. 515 



ized type of organ. If a white-flowered species should give rise to 

 a form with colored flowers, it might well be taken as a degressive 

 movement, or as a retracing of a step once lost, since all flowers were 

 in all probability originally colored. Here again the actual test is 

 hybridization, it being accepted that the retrogressive forms of organs 

 are recessive or latent when crossed with the parental or nearly 

 related types. 



As to latency we can only say that strains of plants do carry 

 capacities of one kind and another for many generations without 

 these particular forms of unit characters showing any activity. 

 Thus a mutant which springs from a parental type and shows a 

 laciniate leaf instead of the ancestral leaf must carry the latter form 

 in a latent condition. The latent character may be awakened from 

 time to time, or may be entirely and permanently inactive. The 

 most easily analyzable examples of latency are seen in hybrids. Thus 

 when the blue-flowered Veronica longifolia was crossed with a white- 

 flowered variety derived from it, the progeny was entirely and con- 

 tinuously blue flowered, except for occasional bud sports. The white- 

 flowered condition was here very evidently in a latent condition. In 

 other instances white-flowered forms have appeared as a recessive, 

 forming one-fourth of the progeny. If qualities have a cytological 

 basis, and we may certainly assume that they do, then it is not easy 

 to speculate intelligently on the probable condition of latent char- 

 acters. 



A very striking feature of mutants consists in the fact that the 

 gross anatomical characters in which they diverge from the parent 

 shows a much wider range of variability around its new norm than 

 does the homologous character in the parental type. Many observa- 

 tions bearing this interpretation have been on record for sometime. 

 Charles Darwin, in his Origin of Species, noted that varieties varied 

 more widely than the closely related species from which they were 

 supposed to be derived, and many other observers have touched in 

 one way or another upon the subject. Doctor Shull's recent re- 

 searches upon this subject led him into the making of exact measure- 

 ments, by which he also finds that the wide fluctuations of the mutant 

 characters are accompanied by a lesser degree of correlation than 

 prevails in the parental forms. Thus, for instance, the leaves of 

 rubyinei'vis not only vary relatively more in width than those of the 

 parental Lamarckiana^ but the proportion between the width and 

 length is not so constant as in the latter. Other organs of these and 

 other species were subjected to exact measurements with similar re- 

 sults, while a large number of recent observations are known which 

 justify the conclusions just stated. Of these, the great variation of 

 the length of the pistil in hTemHtylk in comparison, and the wide 

 fluctuations in laciniate leaves are good examples. Many of the latter 



