Referate. 235 



be found to have kept pace with the advance in knowledge during the last 

 few years. For example, the autlior makes an extended criticism of De Vries' 

 earlier claim that there is a fundamental distinction in the heredity of what 

 he calls elementary species and what he calls varieties. Genetic research 

 may lead to definite distinctions between species and varieties, but De Vries 

 was certainly not in position to make .such a distinction in 1903. The recent 

 work of Brainerd and of Price has sho\vn definitely that characters possessed 

 by different species of Viola and of Lycopersicum do mendelize when crossed. 

 Other work with species crosses has shown that certain characters apparently 

 blend in the Fi-generation. If these characters break up in the Fo-generation, 

 it may be due to the complicated inter-action of a large number of Mendelian 

 characters; if they remain true, it may be because only zygotes possessing 

 certain characters can be formed. On the other hand, there may be true blen- 

 ding characters, whose behavior in inheritance has not been detennined. It 

 is only when light is thrown on these different questions, that genetic 

 distinctions between species and varieties wiU be possible. 



Notwithstanding the fact that there is a probability that many of the 

 authors cited would have anticipated Prof. Holmes in his criticisms had 

 they discussed these matters at the present time, nevertheless he has un- 

 doubtedly served biology? in calling attention to various tentative conclusions 

 from different more or less fragmentary researches, which, when considered 

 together in their relations to the fundamental problems of biology, are either 

 insufficient to support their claims or antagonistic to each other. Whether 

 these bones do not fit the skeleton involved, as the author believes, or 

 whether it is a question of finding the lost pieces and assembling them 

 correctly, must be left for future decision. 



The article touches so many points which the author himself has not 

 systematized in his outline that it is almost impossible for a reviewer to 

 consider them without utilizing too much space; we therefore quote his 

 own conclusion. 



,,In the preceding discussion the attempt has been made to show that the 

 various categories of variations recognized by De Vries and others are not 

 sharply separable either on morphological grounds or by their behavior 

 when subjected to crossing experiments. The attempt was made also to show 

 that neither the facts of variability nor those of Mendelian inheritance give 

 any support to the doctrine of pangens, determinants, or other a.ssumed 

 bearers of unit characters, and that unit characters as elements that can 

 enter or depart from the complex of tendencies that make up an organism 

 probably have no existence. It is evident that variations differ in their 

 stability, but the explanation of this fact may lie in the physiological relations 

 of the variation rather than in hypothetical representative units. WTiether 

 the variations of the discontinuous type have been influential, in any marked 

 degree, in shaping the course of evolution is a question upon which we need 

 much more evidence. Mutations, as we have seen, may be very small 

 affairs. About the only criterion by which they may be recognized is their 

 stability, and even that gives some evidence of being a matter of degree. 

 No limit has been discovered to the minuteness of the stable modifications 

 that may occur, and it may happen that further study will reveal the com- 

 paratively frequent appearance of the very slight variations of this kind. 

 In fact, considerable progress has even now been made in this direction by 

 the study of grains; and the number of more or less stable modifications 

 that are likely to be discovered threatens to overwhelm systematists with 

 the labor of naming and describing them. In many organisms not propa- 



