68 DIE bUMPF- UND WASSERPFLANZEN. 



The two most important forms of discriminate isolation are — 

 first, Sexual Incompatibility, either partial or absolute, between 

 different taxonomic groups, or Physiological Selection; and second, 

 Natural Selection. The importance of indiscriminate selection 

 depends on the fact, pointed out by Mr. Gulick, that no two 

 portions of a species have exactly the same average character, and 

 the initial differences by continually reacting upon each other, and 

 on the environment, ensure increasing divergence as long as the 

 individuals of the two groups are kept from intergenerating. That is 

 to say, the originally indiscriminate selection becomes discriminate. 



Chapters I. and II. are devoted to a discussion of Isolation as a 

 general pi-inciple. In the third we pass on to consider Physio- 

 logical Selection, or "that form of isolation which arises in con- 

 sequence of mutual infertility between the members of any group 

 of organisms and those of all other similarly isolated groups 

 occupying simultaneously the same area." The author summarizes 

 the main points of this theory, and in the next chapter puts forward 

 several kinds of evidence in its favour. These include evidence 

 from geographical distribution. The production of numerous spe- 

 cies in large areas, and the richness in species of dominant genera 

 within those areas, points on which Darwin insisted, are shown to 

 be capable of explanation by the working of physiological selection. 

 Evidence from topographical distribution [i. c. distribution with 

 reference to comparatively small areas), both of species and varieties, 

 and from the working of prepotency, is also adduced, and is of special 

 interest to botanists. 



The last chapter is an historical sketch entitled " A brief history 

 of opinions on Isolation as a factor of organic evolution," and con- 

 cludes with twelve "Articles," which declare that "Natural 

 Selection has been the main, but not the exclusive means of 

 modification," for "without isolation, or the prevention of inter- 

 crossing, organic evolution is in no case possible. Therefore it is 

 Isolation which has been the exclusive means of modification," or, 

 more correctly, the universal condition to it; and finally, "where 

 common areas and polytypic evolution are concerned, the most 

 general and most efficient form of isolation has been the physio- 

 logical." The volume is an eminently suggestive one, especially 

 for botanists, for plants afford the best material for the kind of 

 work which must establish or refute the articles laid down by Mr. 

 Eomanes. ABE 



Die Sumpf- wul Wasserpjianzen. Ihre Beschreibuiir/, Kultiir mul 



Verwnidiing. Von Wilh. Monkemeyer. 8vo, pp. iv, 189, with 



126 figures in the text. Berlin : Schmidt. 1897. 



The object of the present volume is to meet the want of a book 



dealing with water- and damp-loving plants — a want which has 



arisen from the increased use of aquaria, and also of aquatic and 



marsh-plants in landscape-gardening. The author is "Inspector" 



of the Botanic Garden of Leipzig University, and is therefore 



presumably in a position to speak with authority on points of 



