HYBRIDIZATION BETWEEN DIOECIOUS AND HERMAPHRODITE SPECIES IN THE CARYOPHYLLACEAE

are restricted to northeasternmost America and northwesternmost Europe and have not been transported by aid of man over the ocean, Lindroth seems to be of the opinion that their range cannot be explained by aid of the putative landbridge. The present writer agrees, although there is nothing against the hypothesis that a land connection of a similar or even much more compact type has occurred earlier between these regions. Instead, Lindroth accepts Hulten's hypothesis completely and regards these distributions as "renmants of a broken-up area of circumpolar type. " It is possible that this is correct although many botanists doubt it on, as far as can be judged, rather good reasons. In fact, the problem can perhaps be most easily solved by aid of palynological studies and detailed cytogenetic analyses based on the easternmost and westernmost populations of both continents. As is unavoidable in a book of this kind, studying a very complex problem which needs to be enlightened from many different angles, many statements are somewhat doubtful and others may be a bit misleading although not intentionally so, and some data may perhaps be stressed too far to fit the explanations the author believes in. The value of the book as a contribution towards the solution of a delicate and important evolutionary problem is not diminished much by these human fallibilities, but the reader must take care to consult such data with great forethought, especially when outside the special field of the author him-' self. It should also be emphasized that the author does not seem to understand the great significance of cytogenetical studies in such evolutionary problems, and in some places he minimizes judgments made on the most conclusive evidence available for evolutionary classification of taxa. The problem of long-distance dispersal, landbridges, and renmants of broken-up circumpolar areas has received very substantial treatment by Professor Lindroth, and his contribution, though far from supporting the idea of longdistance dispersal, will long remain indispensable for those interested in these questions. But it seems as if the study of these matters is approaching the point at which morphological and geographical investigations alone become insufficient and must be assisted by more detailed experimentation. The recent explanation regarding long-distance dispersal of the so-called American element in the flora of the British Isles seems to have been made very doubtful by the cytological demonstrations of different degrees of polyploidy in American and Irish populations of some of the essential species of this element (Love and Love, 1958). Also, unpublished studies from North America have revealed great heterogeneity in some of the supposedly circumpolar plants. Whenever such detailed experimental studies are started with the animals or plants of the amphiatlantic groups in order to get closer to the truth of their history and origin, the evidence produced by Lindroth for the transatlantic connections certainly will be of utmost importance. The book by Lindroth should be read by all those interested in such problems, and it should be available in every library. Its critical reading will support those who doubt the correctness of the hypothesis of long-distance dispersal over unfrozen seas, but it is not equally certain that it will remove all doubts regarding the explanation of all ranges as renmants of a circumpolar area with a centrum only around the Bering Sea. The biological world is hardly so simple that a single hypothesis can explain all the complexities of the many plants and animals that inhabit the lands around the boreal hemisphere.