DISCOVERY 



331 



Reviews of Books 



Age and Area. A Study in Geographical Distribution 

 and Origin of Species. By Dr. J. C. Willis, F.R.S. 

 With chapters by Hugo De Vries, H. B. Guppy, 

 IMrs. E. M. Reid, and Dr. J. Small. (Cambridge 

 University Press, 14^.) 



The geographical distribution of plants over the earth's 

 surface has long occupied the attention of naturalists, 

 but it was Darwin's treatment of the subject in the 

 Origin of Species which initiated a more scientific study 

 of plant geography. " In considering the distribution 

 of organic beings over the face of the globe," he wrote, 

 " the first great fact which strikes us is, that neither the 

 similarity nor the dissimilarity of the inhabitants of 

 various regions can be accounted for by climatal and other 

 physical conditions." Darivin held that every species 

 began its existence at some one place, whence it gradually 

 spread. Some species w-ere successful travellers, while 

 others lagged behind and never occupied more than a 

 restricted area. The idea of multiple origins, that is, 

 that the same species may have been created indepen- 

 dently at more than one place, was superseded in the minds 

 of most naturalists by that of single centres of creation. 

 As Darwin said, " The simplicity of the view that each 

 species w-as first produced within a single region capti- 

 vates the mind." He also WTote : " As many more 

 individuals of each species are born than can possibly 

 survive ; and as, consequently, there is a frequently 

 recurring struggle for existence, it follows that any being, 

 if it vary however sUghtly in any manner profitable to 

 itself, under the complex and sometimes varying condi- 

 tions of life, will have a better chance of surviving, and 

 thus be naturally selected. The preservation of favour- 

 able varieties, and the rejection of injurious varieties, 

 I call Natural Selection." 



The doctrine of natural selection in Darwin's mind 

 was intimately connected w-ith geographical distribution. 

 Dr. Willis, as he tells us in the Preface, was formerly a 

 " pupil of the strictest school of Natural Selection " ; 

 but, he adds, a course of " independent observation of 

 nature," extending over several years, led him to shake 

 oS the trammels of the theory and compelled him to 

 profess his belief that " Age as an explanation of spread 

 is enormously simpler than natural selection." An 

 analysis of the flora of Ceylon and, subsequently, of the 

 floras of other countries, led to the enunciation of the 

 hvpothesis which is conveniently spoken of as Age and 

 Area. In the volume before us Dr. Willis summarises 

 and amplifies views previously published in various 

 scientific journals. It is impossible in a short review- to 

 deal adequately with the many interesting problems raised 

 by the author ; one can only draw attention to the salient 

 features of the hypothesis and offer a few criticisms. 



In Part I he deals with the present position of Age 

 and Area. In the introductory chapter he wTites : 

 " For sixty years we have been under the wonderful 

 fascination of the theory of evolution by means of 

 infinitesimal variations, or minute changes of character 



from individual to individual." So far as I am aware, 

 Darwin never speaks of variations as " infinitesimal." 

 Seeing that Dr. Willis is a confirmed unbeliever in 

 Natural Selection, it would have been better to quote 

 Darwin's words and thus make clear the precise meaning 

 which Darwin attached to that expression. Willis adds 

 that the distribution of any species is governed by the 

 interaction of many factors and physical barriers ; but 

 while the various factors will be likely to act with some 

 uniformity only on a group of allied species, age puUs 

 all alike. The area occupied is largely the result of age : 

 widely spread species are older than species confined to 

 a small area. It is true that a recently evolved plant 

 or animal has had insufficient time to travel far from its 

 original home ; but it is equally true that many plants 

 and animals of restricted range formerly occupied a 

 larger territory-. In other words, size of area is not a 

 safe criterion of age. To get to the bottom of the 

 history of organisms, we must consider the evidence of 

 the rocks and take into account distribution in the past 

 as well as in the present. The study of fossil plants 

 shows unmistakably that many genera that are now 

 confined within narrow geographical limits are sur- 

 vivals, and not j-oung beginners. Dr. Willis admits that 

 there are exceptions to his general rule ; but he con- 

 siders their number insignificant. "Another popular 

 theory," he says, " about localised species like these 

 Ceylon endemics [species, genera, or other groups 

 confined to a small area] is still strongly held ... it 

 is to the effect that species on very small areas are really 

 in process of djdng out. This hypothesis is supposed 

 to be supported by the facts of fossil botany, which 

 unquestionably proves that many species have existed 

 in the past and no longer occur in the world to-day." 

 The point is, not that fossil botany proves the former 

 existence of species no longer represented in present-day 

 floras — a fact universally admitted — but that it is no 

 mere supposition that it also teaches us that many 

 genera of plants now confined to small areas are very 

 old types and in former ages had a wide distribution. 

 There are many, not few, such genera. He deprecates 

 conclusions based on individuals ; groups of ten species 

 at least must be considered. But all facts must be taken 

 into account in our endeavour to discover the truth. 



Chapters are included on the dispersal of plants into 

 new areas ; the introduction and spread of foreign 

 species ; acchmatisation ; and other subjects. Part II 

 is devoted to the application of Age and Area to the flora 

 of the world, and its implications. In 1912 the author 

 noticed that of the 1,028 genera in the flora of Ceylon, 

 there are 573 genera with one species each, 176 with two 

 species, 85 with three species, and so on. These numbers 

 clearly show that genera with one and two species are 

 much more numerous than genera with more species. 

 An analysis of floras of many, both large and small, 

 regions revealed the same state of afiairs. If the genera 

 of different floras are arranged according to the number 

 of species they contain and the arrangement is expressed 

 by curves, these exhibit a striking uniformity. The 

 same type of " hollow curve " was obtained by treating 



